Add 'align' option to figure shortcode (adityatelange#256)

Usage: 

{{< figure align=center src="image.jpg" >}}

Parameter values:
  - 'center': align image and captions to center of page
  - anything else neglected

* Appends "#center" to the image's URL in order to utilize existing CSS
rules for centering images.

assets/css/post-single.css

@@ -242,6 +242,10 @@
     margin: auto
 }
 
+.post-content figure.align-center {
+	text-align: center;
+}
+
 .post-content figure > figcaption {
     color: var(--primary);
     font-size: 16px;

layouts/shortcodes/figure.html

@@ -1,8 +1,11 @@
-<figure{{ with .Get "class" }} class="{{ . }}"{{ end }}>
+<figure{{ if or (.Get "class") (eq (.Get "align") "center") }} class="
+           {{- if eq (.Get "align") "center" }}align-center {{ end }}
+           {{- with .Get "class" }}{{ . }}{{- end }}"
+{{- end -}}>
     {{- if .Get "link" -}}
         <a href="{{ .Get "link" }}"{{ with .Get "target" }} target="{{ . }}"{{ end }}{{ with .Get "rel" }} rel="{{ . }}"{{ end }}>
     {{- end }}
-    <img src="{{ .Get "src" }}"
+    <img src="{{ .Get "src" }}{{- if eq (.Get "align") "center" }}#center{{- end }}"
          {{- if or (.Get "alt") (.Get "caption") }}
          alt="{{ with .Get "alt" }}{{ . }}{{ else }}{{ .Get "caption" | markdownify| plainify }}{{ end }}"
          {{- end -}}