Add feature to remove matching brackets

This is a new feature for removing a pair of brackets.
It is currently bound to `ctrl-backspace`. I don’t have access to a Windows or a Linux machine and I’m not sure if `ctrl-backspace` is bound to anything on those platforms.

![Remove matching brackets screen grab](http://i.imgur.com/ljCkYjb.gif)
`mutateSelectedText` is called once for each selection, do you intend to support multiple cursors?

If you do want to support then, then the function to `mutateSelectedText` should use the passed in `Selection` object and operate on it directly, it as `selectLeft`, `getText`, and `backspace` methods on it.