Add --pause option to `pull rebase` to pause before pushing (useful for testing)

This issue comes from #29 and is particularly useful to test rebased pull requests just before they are actually pushed to the blessed repository and closed.

Basically if you use `--pause`, then after the rebase was successful, you'll get a prompt just like when there were some conflicts, and you can use `--continue` to finish the rebasing (pushing and closing the issue).