Teach rustdoc to syntax highlight

[brson]

We can probably port the js code from doc/lib to Rust. It's really small.

[thehydroimpulse]

@brson For syntax highlighting, we could pipe the code snippets to pygments while generating the docs, or we could do the highlighting on the client with something like highlight.js. The former option seems better, and is what I'm currently using for prototyping.