Support for Documenter.jl style code blocks

[kescobo]

It would be awesome if code fences that use Documenter.jl's syntax could be highlighted properly. Eg.

```@example
x = 2 + 2
```

or

```@repl 1
x = 2 + 2
```

It looks like the code is here, and uses some form of regular expressions? I'd be happy to try a PR, but that looks like jibberish to me. Are there any useful resources on whatever that syntax is?