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
```

首先跟踪 `dataSource: milestones` 路径以及 gren 配置的处理方式。检查 release 是如何完成的，以及如何访问 milestones，然后找出 release 行为现有的测试入口。完成标准是：启用可选设置 `closeMilestoneAfterRelease: true` 后，相关 milestone 会在 release 之后关闭，同时不改变默认行为。

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