wip: inline & display math

I'm working to support [MathJax](http://www.mathjax.org/).

Output tag format:
http://docs.mathjax.org/en/latest/model.html#how-mathematics-is-stored-in-the-page

The idea for now:

* `$$ ... $$` => inline math
* `$$$ ... $$$` => display math

The code I wrote is mostly monkey-patching. Could anyone help get it settled?