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)
I did not plan on supporting multiple cursors. I really don’t see a use case for it. I’m open to all suggestions though :)

Should I be doing this some other way?