Teach marge-bot how to backport merge requests

Hi,

Something that'd be really neat would be to use marge-bot not only for rebasing and merging merge requests, but also backport to selected stable branches.

I imagine this would work by marge-bot doing the following.

(0. Wait for merge-request proper to land)

1. Creating a new backport branch
2. Rebasing that backport branch on top of the stable branch
3. Creating a merge request for said branch
4. Automatically assign itslef to said merge request
5. Merge as usual

I imagine the "user interface" could be doing things like "@ marge-bot, backport stable-branch-1-2-3"

Does this sound like something within scope of marge-bot's abilities?