Add rmbranch alias to git.config

The new alias runs `git branch -d <branchname>` and then runs `git
push origin --delete`.  Removes the branch local and remotely if changes
have been merged upstream.

Passes `-p` flag which (from git-branch man):
> Delete a branch. The branch must be fully merged in its upstream branch,
> or in HEAD if no upstream was set with --track or
>           --set-upstream.

More information can be found [here](http://stackoverflow.com/questions/2003505/how-do-i-delete-a-git-branch-both-locally-and-in-github)