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




You could just use the presence of MathJax to decide and not have a separate variable, maybe?
Is there any reason why we wouldn't want to use MathJax if it's loaded? Isn't it more complete? The only reason I can think of would be if we liked the KaTeX rendering output more.