Font-lock code blocks according to mode.

The correct way to do this would be to deduce the major mode thusly:

1. Is there a `<!-- language: * -->` line above our code block? If so, use that mode. (We could create an alist here if necessary.)
2. Query SE for the language relation based on tags. There doesn't seem to be a way to do this yet, but if I'm not mistaken this is the way it's done normally.
3. Use this structure to do the final guessing if necessary.