Add custom syntax for adding classes in Markdown

To support this kind of custom SourceJS markup:

```
<a href="/docs/spec" class="source_a_hl">Read more about Spec pages here.</a>
<div class="source_note">
    After initialization, you get `sourcejs/user` folder, which is the place for all your custom content. All new Specs and configuration of main engine must be done there.
</div>
```
