Have option for Marge to automatically remove WIP status of branches to merge

Created by: trickl

By default, branches named "wip_" will get rejected by Marge as the merge request that is generated will start with "wip" and be marked in progress.

https://docs.gitlab.com/ce/user/project/merge_requests/work_in_progress_merge_requests.html

Typically developers will work on a development branch and label it as "wip_" to distinguish it from production branches, then decide to Marge it - in which case they will expect Marge to remove the WIP tag. Presently, their work flow would have to be:

1. Create development branch (wip...)
2. Submit MR
3. Watch MR get rejected unexpected by Marge as its WIP.
4. Rename their branch after figuring out the problem.
5. Resubmit

This is less than ideal and could be solved if Marge automatically removed the WIP tag. As some teams might actually want this protection in place, I would make it an optional setting of Marge so teams can decide the behaviour they want.