Option to fallback to HighlightJS

This adds a command-line option to specify the highlighter used.

By default, Pygments will be used but if it is not detected on the system or the user has specified "[highlightjs](https://github.com/isagalaev/highlight.js)", that will be used instead. The default CSS now contains styles for both types of marked-up code. 

I've [submitted a pull request](https://github.com/isagalaev/highlight.js/pull/138) to highlightJS to improve the detection and highlighting of coffee-script and include a Docco style.

This should maintain compatibility with existing usage and any custom CSS while making the python requirement optional.