Comment on the issues and PRs after release

```
// ship.js.config
{
commentOnGitHubAfterRelease: true, // this name is totally temporary
}
```

With `commentOnGitHubAfterRelease: true`, at the end of `yarn release:trigger`, Ship.js will go to all the related PRs and leave comments like `This PR is included in the recent release vx.y.z`.

It seems semantic-release does this, too.

example: https://github.com/downshift-js/downshift/pull/971#issuecomment-599040696

Start by tracing the yarn release:trigger flow and the ship.js.config option commentOnGitHubAfterRelease. Identify how the release finds related PRs, then compare the requested behavior with the linked Downshift example and semantic-release; done means each related PR receives a versioned release comment.

Written by the indexing model from the issue text.