Add highlighted code block rules

src/gfm.js

@@ -1,9 +1,15 @@
+import highlightedCodeBlock from './highlighted-code-block'
 import strikethrough from './strikethrough'
 import tables from './tables'
 import taskListItems from './task-list-items'
 
 function gfm (turndownService) {
-  turndownService.use([strikethrough, tables, taskListItems])
+  turndownService.use([
+    highlightedCodeBlock,
+    strikethrough,
+    tables,
+    taskListItems
+  ])
 }
 
-export { gfm, strikethrough, tables, taskListItems }
+export { gfm, highlightedCodeBlock, strikethrough, tables, taskListItems }

src/highlighted-code-block.js

@@ -0,0 +1,25 @@
+var highlightRegExp = /highlight-(?:text|source)-([a-z0-9]+)/
+
+export default function highlightedCodeBlock (turndownService) {
+  turndownService.addRule('highlightedCodeBlock', {
+    filter: function (node) {
+      var firstChild = node.firstChild
+      return (
+        node.nodeName === 'DIV' &&
+        highlightRegExp.test(node.className) &&
+        firstChild &&
+        firstChild.nodeName === 'PRE'
+      )
+    },
+    replacement: function (content, node, options) {
+      var className = node.className || ''
+      var language = (className.match(highlightRegExp) || [null, ''])[1]
+
+      return (
+        '\n\n' + options.fence + language + '\n' +
+        node.firstChild.textContent +
+        '\n' + options.fence + '\n\n'
+      )
+    }
+  })
+}

test/index.html

@@ -273,6 +273,28 @@
 | --- |</pre>
 </div>
 
+<div class="case" data-name="highlighted code block with html">
+  <div class="input">
+    <div class="highlight highlight-text-html-basic">
+      <pre>&lt;<span class="pl-ent">p</span>&gt;Hello world&lt;/<span class="pl-ent">p</span>&gt;</pre>
+    </div>
+  </div>
+  <pre class="expected">```html
+&lt;p&gt;Hello world&lt;/p&gt;
+```</pre>
+</div>
+
+<div class="case" data-name="highlighted code block with js">
+  <div class="input">
+    <div class="highlight highlight-source-js">
+      <pre>;(<span class="pl-k">function</span> () {})()</pre>
+    </div>
+  </div>
+  <pre class="expected">```js
+;(function () {})()
+```</pre>
+</div>
+
 <!-- /TEST CASES -->
 
 <script src="turndown-plugin-gfm-test.browser.js"></script>