Close a milestone after a release

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.
```yml
closeMilestoneAfterRelease: true
```

Start by tracing the `dataSource: milestones` path and the gren config handling. Check how a release is completed and how milestones are accessed, then identify the existing test entry points for release behavior. Done means an opt-in `closeMilestoneAfterRelease: true` setting closes the relevant milestone after release without changing the default behavior.

Written by the indexing model from the issue text.