Add ruby string interpolation

Addresses #3
You probably want to use a scope selector for this:

```coffee
{ScopeSelector} = require 'first-mate'
interpolatedRubySelector = new ScopeSelect('interpolated.ruby')
if interpolatedRubySelector.matches(@editor.getCursorScopes())
  # it is interpolated ruby
```

You can see examples here: https://github.com/atom/bracket-matcher/blob/f7c9b9fe30b169aaaaa752ca3662e92f87ea37ad/lib/tag-finder.coffee#L13-L29