Integrate with GitHub

[jonatanklosko]

It would be great to integrate this grammar for GitHub highlighting. Note that this grammar is not likely to be used in files, but it would be useful for highlighting Markdown snippets, like this:

We first compute square of the numbers:

```iex
iex> list = Enum.map([1, 2, 3], fn x -> x * x end)
[1, 4, 9]
```

And that's how we would sum them up:

```iex
iex> Enum.sum(list)
14
```

@patrickt please let us know if you're open to integrating more specific grammars like this one.