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)
Yeah, you'll just want to return early when more than one cursor exists, I can merge this and just update it if that is cool with you.