Add rmbranch as a git alias.

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.