Adding a key binding for this extension

Hi,

I really like your extension and found I was auto indenting my code quite a bit. A keyboard shortcut seemed like the obvious thing to do.

I had a chat with some peeps on Google Groups https://groups.google.com/forum/#!topic/brackets-dev/pFalnjpPg6Y

The https://bitbucket.org/sacah/brackets-key-remapper extension could do the same job with a bit of setup, but we decided a default key binding is fine and users can remap it later if they wanted to.

cheers,
Andrew