Support pull requests without associated issues

[motiz88]

Pull requests have all the discussion, tagging, search, project management, and other features of issues. But often, in a gren-enabled project, I'm still forced to create a separate issue when I've already made the PR that would close it (e.g. for a previously-unreported problem) because otherwise, the release notes would be incomplete. This adds overhead to what can be a seamless GitHub workflow.

The solution: I think gren should treat merged PRs as their own, self-closing issues if they don't explicitly close any other issue.

[alexcanessa]

I've chatted with GitHub APIs team and there is no way in the APIs to detect whether a PR closes any issues.

They're actually suggesting a RegExp against the keywords.

Still, something to address.