Update to latest master of Hoedown to get Mathjax support

[Rustdoc wants mathjax support](https://github.com/rust-lang/rust/issues/16300). Client-side impl is trivial, but the server-side is tricky. Thankfully, Hoedown just merged in a PR that adds mathjax support!

When given the `--math` flag, `$$math$$` will be parsed as mathjax in a context-sensitive way. If it is part of a block, it will become inline math. If it's a whole block, it will become block math. We can opt into non-sensitive usage with another flag, but that makes `$math$` inline, and I don't think we want to have a single `$` be significant.

The rest of the commits seem to be bugfixes, cleanup, and testing, but I haven't rigorously reviewed them.