Markdown code class doesn’t follow convention

[WHATWG suggests](http://www.whatwg.org/specs/web-apps/current-work/multipage/text-level-semantics.html#the-code-element) using `language-` as the prefix for the class name on code blocks. This hasn’t been changed in Marked yet, so writing:

<pre class="markdown">
```json
"hello-world": {
  "title": "the-title"
}
```
</pre>

generates HTML with the class `lang-json` instead of `language-json`.