Markdown code highlighting

Allows the template to expose a highlight function (which takes a code and lang param) to highlight code samples in markdown (when the marked parser is used) similarly to code examples in jsdoc comments. The change is covered by unit tests.

Before:

![markdown highlighting before](https://f.cloud.github.com/assets/1655575/507609/99d5a26a-bd73-11e2-90ae-89715752acd1.png)

After:

![markdown highlighting after](https://f.cloud.github.com/assets/1655575/507610/a1d2d208-bd73-11e2-96d7-5085cb6bd610.png)

I couldn't figure out how to do this with the evilstreak parser, but maybe someone else would be able to do that at some point in the future.

It's completely backwards compatible: if the template doesn't expose a highlight function the behavior is as it was before this change.