feat(subParsers/githubCodeBlock): add extra language class to conform to html5 spec

dist/showdown.js

@@ -1396,7 +1396,7 @@ showdown.subParser('githubCodeBlocks', function (text, options, globals) {
     codeblock = codeblock.replace(/^\n+/g, ''); // trim leading newlines
     codeblock = codeblock.replace(/\n+$/g, ''); // trim trailing whitespace
 
-    codeblock = '<pre><code' + (language ? ' class="' + language + '"' : '') + '>' + codeblock + end + '</code></pre>';
+    codeblock = '<pre><code' + (language ? ' class="' + language + ' language-' + language + '"' : '') + '>' + codeblock + end + '</code></pre>';
 
     return showdown.subParser('hashBlock')(codeblock, options, globals);
   });

src/subParsers/githubCodeBlocks.js

@@ -32,7 +32,7 @@ showdown.subParser('githubCodeBlocks', function (text, options, globals) {
     codeblock = codeblock.replace(/^\n+/g, ''); // trim leading newlines
     codeblock = codeblock.replace(/\n+$/g, ''); // trim trailing whitespace
 
-    codeblock = '<pre><code' + (language ? ' class="' + language + '"' : '') + '>' + codeblock + end + '</code></pre>';
+    codeblock = '<pre><code' + (language ? ' class="' + language + ' language-' + language + '"' : '') + '>' + codeblock + end + '</code></pre>';
 
     return showdown.subParser('hashBlock')(codeblock, options, globals);
   });

test/cases/github-style-codeblock.html

@@ -7,5 +7,5 @@
 
 <p>And some HTML</p>
 
-<pre><code class="html">&lt;div&gt;HTML!&lt;/div&gt;
+<pre><code class="html language-html">&lt;div&gt;HTML!&lt;/div&gt;
 </code></pre>