Support pull requests without associated issues

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.