Add pipeline step to close merge request

Hi!

It would be neat to be able to close a GitLab merge request inside a Jenkins pipeline.

Some background: I'm building a build queue for the company I work at that will automatically rebase, merge and push semi-linear merge commits (GitLabs nomenclature).
When a rebase is needed the new commits will have new SHA:s, which means that GitLab won't understand that the branch is actually merged (kinda). So I would like to be able to close them instead. :)