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




If we do it that way, I think we can do it like this:

```coffee
rendered = $sanitize $filter('converter') value
try
  rendered = renderMath rendered
catch
  MathJax.Hub.Queue(['Typeset', MathJax.Hub, output])
scope.rendered = $sce.trustAsHtml rendered
```