Add "checkout" command to checkout new and existing branches

[robbawebba]

Currently there are no registered commands for checking out new and existing branches. Although the status bar tile is a pretty easy way to check out different branches, it would be nice to perform the same actions in Atom without leaving the keyboard.

I noticed there are already callback functions for checking out and creating new branches in the branch menu view, so it seems possible to register commands that use those existing callback functions (or something similar).

[robbawebba]

@BinaryMuse If you don't mind, I'd like to take this feature on!

[BinaryMuse]

@robbawebba You got it!