Close a milestone after a release

[tlinkowski]

First of all, thanks for a great app!

I'd like to propose the following new feature:

• when we have dataSource set to milestones, gren could close the milestone after a release.

Right now, I have to do it manually, and it's easy to forget.

In order not to break backwards compatibility, it could be an opt-in behavior through a gren config file, e.g.

closeMilestoneAfterRelease: true