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




Since you are only supporting TeX-like input, this should be `url:"//cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML-full` to be at least a little bit more efficient.