Figure out how to handle being embedded

[alerque]

One cool feature of vim's syntax highlighting is the way it can embed an entire set of syntax rules in a group for another language. This is most often employed for Markdown the other way: fenced code blocks can contain other languages. But it's possible to use the other way around and many languages (Rust and Python documentation strings, etc.) can embed markdown.

The way we're handling passing the full buffer contents makes this system blow up. Either we need to figure out a way to handle it or document that it doesn't work like that.