handle examples in docs

[droundy]

It would be lovely to have proper rust highlighting and handling in doc examples. e.g. when editing

//! ```
//! let x = 1;
//! ```

I would love to have rust indentation and syntax highlighting on the example code.

[nikomatsakis]

cc @pnkfelix didn't you have some setup that could somehow do this?

[appetrosyan]

This should be similar to how the lisp symbols are highlighted in the regular elisp buffers.