Add mathjax.

Adding the `mathjax: true` metadata to any page will include the
associated JS. Then use $<math>$ for inline math or $$<math>$$ for a
block. See http://www.forkosh.com/mimetextutorial.html for help with
LaTex math. The math is formatted using MathJax
(https://github.com/mathjax/MathJax).