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




Fair enough. Then keep it a per-annotation thing.

However, you should definitely heed my comment from last time about passing the element to the MathJax Hub so that only the current annotation has its math rendered. Right now you're re-rendering all the math in all annotations for every annotation, so it's no surprise you're seeing it become sluggish. If you have 10 annotations on the page you're currently rendering 10 times as much math as you need to.