Default to GitHub Flavored Markdown

- Configure Turndown for GFM per [docs][1]
- Use [a fork of turndown-plugin-gfm][2] due to [issues with the
  original turndown-plugin-gfm][3]
- Remove custom strikethrough logic which is now redundant due to GFM
  plugin

[1]: https://github.com/domchristie/turndown#usepluginarray
[2]: https://www.npmjs.com/package/joplin-turndown-plugin-gfm
[3]: mixmark-io/turndown-plugin-gfm#3

package.json

@@ -48,6 +48,7 @@
     "webpack-cli": "^4.6.0"
   },
   "dependencies": {
+    "joplin-turndown-plugin-gfm": "^1.0.12",
     "turndown": "^7.0.0"
   }
 }

src/background.js

@@ -1,4 +1,5 @@
 import TurndownService from 'turndown'
+import { gfm } from 'joplin-turndown-plugin-gfm'
 
 // Uncomment to bust any cached settings while debugging
 // localStorage.clear();
@@ -24,7 +25,7 @@ import TurndownService from 'turndown'
    */
   const settings = new Store('settings', {
     // Render these elements into markdown output as HTML
-    keep_elements: 'table, small',
+    keep_elements: '',
     // Do not consider these elements when rendering markdown output
     delete_elements:
       'script, style, title, noscript, canvas, embed, object, param, svg, source, iframe',
@@ -42,19 +43,19 @@ import TurndownService from 'turndown'
   }
   let turndownService = new TurndownService(turndownOptions)
 
+  // Use GitHub Flavored Markdown Turndown plugin
+  // Fop better table and strikethrough support
+
+  // Use the gfm plugin
+  turndownService.use(gfm)
+
+  // Use the table and strikethrough plugins only
+  // turndownService.use([tables, strikethrough])
+
   // Apply settings for elements to keep and delete
   turndownService.remove(deleteElements)
   turndownService.keep(keepElements)
 
-  // Suport ~strikethrough~
-  // Taken from example at https://github.com/domchristie/turndown#methods
-  turndownService.addRule('strikethrough', {
-    filter: ['del', 's', 'strike'],
-    replacement: function (content) {
-      return '~' + content + '~'
-    },
-  })
-
   // Fence all <pre> elements
   // GitHub, for example, does not use <code> elements in their code blocks.
   // See https://github.com/domchristie/turndown/issues/192

src/options_custom/manifest.js

@@ -8,7 +8,7 @@ this.manifest = {
             "name": "keep_elements",
             "type": "textarea",
             "text": "div, span, table, del, etc",
-            "default": "table, small"
+            "default": ""
         },
         {
             "tab": "Options",