Allow Marge to re-assign to last assignee after merge

[snim2]

In GitLab merge request analytics there is a table which gives some useful metrics about MRs in a project:

However, if the project uses marge-bot to handle merging, the final column will always show the marge-bot user, and not the previous assignee (presumably the MR reviewer).

This information would be much more useful if Marge re-assigned each MR back to its previous assignee, after the MR has been merged.

This likely needs a new CLI switch, and documentation, to make the feature optional.

I would be willing to contribute this feature, if there's interest in it.