Add math support

This PR adds math support. It uses a new and very light math rendering library from Khan Academy called [KaTex](http://khan.github.io/KaTeX/).

However, since the library is new and [lacking support for some Tex commands](https://news.ycombinator.com/item?id=8320439), I've also implemented a MathJax fallback which will load MathJax from a CDN if KaTex throws an error in rendering. Khan Academy is also doing something similar apparently. 

Dokku = https://math-support.dokku.hypothes.is




`$timeout` takes a third parameter that lets you execute it outside the digest cycle.

```coffee
$timeout (-> MathJax?.Hub.Queue ['Typeset', MathJax.Hub, output]), 0, false
```

That will save some cycles.