Support for parsing codeblocks with highlighting.

This adds support for parsing markdown-style codeblocks that can be highlighted. The syntax has triple backticks and the name of the language to highlight. Currently, the html backend generates a <pre class="language-LANG"> element. The other backends just display the code.

The actual highlighting still has to be done. During zurihac we discussed the idea of bundling highlightjs which can pick up the <pre> tags.