Implement extras extension

Enables inclusion of these HTML elements in Markdown:

- Inserted Text (++inserted++)
- Mark Text (==marked==)
- Subscript (H~2~O)
- Superscript (1^st^)

Closes #12

extras/_test/insert.txt

@@ -0,0 +1,57 @@
+1
+//- - - - - - - - -//
+++Hi++ Hello, world!
+//- - - - - - - - -//
+<p><ins>Hi</ins> Hello, world!</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+2
+//- - - - - - - - -//
+This ++has a
+
+new paragraph++.
+//- - - - - - - - -//
+<p>This ++has a</p>
+<p>new paragraph++.</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+3
+//- - - - - - - - -//
+x ++++foo++ bar++
+//- - - - - - - - -//
+<p>x <ins><ins>foo</ins> bar</ins></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+4
+//- - - - - - - - -//
+x ++foo ++bar++++
+//- - - - - - - - -//
+<p>x <ins>foo <ins>bar</ins></ins></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+5
+//- - - - - - - - -//
+x ++++foo++++
+//- - - - - - - - -//
+<p>x <ins><ins>foo</ins></ins></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+6
+//- - - - - - - - -//
+**++test**++
+
+++**test++**
+//- - - - - - - - -//
+<p><strong>++test</strong>++</p>
+<p><ins>**test</ins>**</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+7
+//- - - - - - - - -//
+[++link]()++
+
+++[link++]()
+//- - - - - - - - -//
+<p><a href="">++link</a>++</p>
+<p>++<a href="">link++</a></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//

extras/_test/mark.txt

@@ -0,0 +1,57 @@
+1
+//- - - - - - - - -//
+==Hello==, world!
+//- - - - - - - - -//
+<p><mark>Hello</mark>, world!</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+2
+//- - - - - - - - -//
+This mark ==has a
+
+new paragraph==.
+//- - - - - - - - -//
+<p>This mark ==has a</p>
+<p>new paragraph==.</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+3
+//- - - - - - - - -//
+x ====foo== bar==
+//- - - - - - - - -//
+<p>x <mark><mark>foo</mark> bar</mark></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+4
+//- - - - - - - - -//
+x ==foo ==bar====
+//- - - - - - - - -//
+<p>x <mark>foo <mark>bar</mark></mark></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+5
+//- - - - - - - - -//
+x ====foo====
+//- - - - - - - - -//
+<p>x <mark><mark>foo</mark></mark></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+6
+//- - - - - - - - -//
+**==test**==
+
+==**test==**
+//- - - - - - - - -//
+<p><strong>==test</strong>==</p>
+<p><mark>**test</mark>**</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+7
+//- - - - - - - - -//
+[==link]()==
+
+==[link==]()
+//- - - - - - - - -//
+<p><a href="">==link</a>==</p>
+<p>==<a href="">link==</a></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//

extras/_test/subscript.txt

@@ -0,0 +1,62 @@
+1: Surrounded by tildes
+//- - - - - - - - -//
+~foo~
+//- - - - - - - - -//
+<p><sub>foo</sub></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+2: Formula with tildes in the middle
+//- - - - - - - - -//
+H~2~O
+//- - - - - - - - -//
+<p>H<sub>2</sub>O</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+3: Indices
+//- - - - - - - - -//
+x~i~ + x~j~
+//- - - - - - - - -//
+<p>x<sub>i</sub> + x<sub>j</sub></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+4: Escaped tilde
+//- - - - - - - - -//
+~foo\~
+//- - - - - - - - -//
+<p>~foo~</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+5: Non-breaking space entity
+//- - - - - - - - -//
+~foo&nbsp;bar~
+//- - - - - - - - -//
+<p><sub>foo bar</sub></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+6: Non-breaking space UTF-8
+//- - - - - - - - -//
+~foo bar~
+//- - - - - - - - -//
+<p><sub>foo bar</sub></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+7: Mixed subscript and strikethrough
+//- - - - - - - - -//
+~~x~foobar~~~
+//- - - - - - - - -//
+<p><del>x<sub>foobar</sub></del></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+8: Tildes in the middle and text before
+//- - - - - - - - -//
+text H~2~O
+//- - - - - - - - -//
+<p>text H<sub>2</sub>O</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+9: Tildes in the middle and text after
+//- - - - - - - - -//
+H~2~O text
+//- - - - - - - - -//
+<p>H<sub>2</sub>O text</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//

extras/_test/superscript.txt

@@ -0,0 +1,69 @@
+1: Surrounded by cares
+//- - - - - - - - -//
+^foo^
+//- - - - - - - - -//
+<p><sup>foo</sup></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+2: Ordinal indicator
+//- - - - - - - - -//
+2^nd^
+//- - - - - - - - -//
+<p>2<sup>nd</sup></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+3: Powers
+//- - - - - - - - -//
+x^2^ + x^5^
+//- - - - - - - - -//
+<p>x<sup>2</sup> + x<sup>5</sup></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+4: Escaped care
+//- - - - - - - - -//
+^foo\^
+//- - - - - - - - -//
+<p>^foo^</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+5: : Cares surround text with a non-breaking space entity
+//- - - - - - - - -//
+^foo&nbsp;bar^
+//- - - - - - - - -//
+<p><sup>foo bar</sup></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+6: Cares surround text Surround with a non-breaking space (UTF-8)
+//- - - - - - - - -//
+^foo bar^
+//- - - - - - - - -//
+<p><sup>foo bar</sup></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+7: Cares in the middle and text before
+//- - - - - - - - -//
+text C^foo^C
+//- - - - - - - - -//
+<p>text C<sup>foo</sup>C</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+8: Cares in the middle and text after
+//- - - - - - - - -//
+C^foo^C text
+//- - - - - - - - -//
+<p>C<sup>foo</sup>C text</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+9: Wrong markers in LaTeX style notation should be left untouched
+//- - - - - - - - -//
+x^2 + x^3
+//- - - - - - - - -//
+<p>x^2 + x^3</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+10: Footnote markers should be left untouched
+//- - - - - - - - -//
+text[^1] text[^2]
+//- - - - - - - - -//
+<p>text[^1] text[^2]</p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//

extras/ast/inline.go

@@ -0,0 +1,106 @@
+package ast
+
+import (
+	gast "github.com/yuin/goldmark/ast"
+)
+
+type InlineTagType int
+
+const (
+	Superscript InlineTagType = iota + 1
+	Subscript
+	Insert
+	Mark
+)
+
+type InlineTag struct {
+	TagType           InlineTagType
+	Char              byte
+	Number            int
+	Html              string
+	WhitespaceAllowed bool
+	ParsePriority     int
+	RenderPriority    int
+}
+
+var SuperscriptTag = InlineTag{
+	TagType:           Superscript,
+	Char:              '^',
+	Number:            1,
+	Html:              "sup",
+	WhitespaceAllowed: false,
+	ParsePriority:     600,
+	RenderPriority:    600,
+}
+
+var SubscriptTag = InlineTag{
+	TagType:           Subscript,
+	Char:              '~',
+	Number:            1,
+	Html:              "sub",
+	WhitespaceAllowed: false,
+	ParsePriority:     602,
+	RenderPriority:    602,
+}
+
+var InsertTag = InlineTag{
+	TagType:           Insert,
+	Char:              '+',
+	Number:            2,
+	Html:              "ins",
+	WhitespaceAllowed: true,
+	ParsePriority:     501,
+	RenderPriority:    501,
+}
+
+var MarkTag = InlineTag{
+	TagType:           Mark,
+	Char:              '=',
+	Number:            2,
+	Html:              "mark",
+	WhitespaceAllowed: true,
+	ParsePriority:     550,
+	RenderPriority:    550,
+}
+
+type InlineTagNode struct {
+	gast.BaseInline
+
+	InlineTag
+}
+
+func NewInlineTag(tag InlineTag) *InlineTagNode {
+	return &InlineTagNode{
+		BaseInline: gast.BaseInline{},
+
+		InlineTag: tag,
+	}
+}
+
+var KindSuperscript = gast.NewNodeKind("Superscript")
+var KindSubscript = gast.NewNodeKind("Subscript")
+var KindInsert = gast.NewNodeKind("Insert")
+var KindMark = gast.NewNodeKind("Mark")
+
+func NewInlineTagNodeKind(tag InlineTagType) gast.NodeKind {
+	var kind gast.NodeKind
+	switch tag {
+	case Superscript:
+		kind = KindSuperscript
+	case Subscript:
+		kind = KindSubscript
+	case Insert:
+		kind = KindInsert
+	case Mark:
+		kind = KindMark
+	}
+	return kind
+}
+
+func (n *InlineTagNode) Kind() gast.NodeKind {
+	return NewInlineTagNodeKind(n.TagType)
+}
+
+func (n *InlineTagNode) Dump(source []byte, level int) {
+	gast.DumpHelper(n, source, level, nil, nil)
+}

extras/go.mod

@@ -0,0 +1,5 @@
+module github.com/gohugoio/hugo-goldmark-extensions/extras
+
+go 1.20
+
+require github.com/yuin/goldmark v1.7.1

extras/go.sum

@@ -0,0 +1,4 @@
+github.com/yuin/goldmark v1.7.0 h1:EfOIvIMZIzHdB/R/zVrikYLPPwJlfMcNczJFMs1m6sA=
+github.com/yuin/goldmark v1.7.0/go.mod h1:uzxRWxtg69N339t3louHJ7+O03ezfj6PlliRlaOzY1E=
+github.com/yuin/goldmark v1.7.1 h1:3bajkSilaCbjdKVsKdZjZCLBNPL9pYzrCakKaf4U49U=
+github.com/yuin/goldmark v1.7.1/go.mod h1:uzxRWxtg69N339t3louHJ7+O03ezfj6PlliRlaOzY1E=