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.

首先追蹤為此 Node 模組描述的 release note 產生流程，尤其要了解它如何從 tag 收集 issue 或 commit，以及如何處理已合併的 pull request。當一個未明確關閉其他 issue 的已合併 PR 可以作為自己的自動關閉 release note 項目出現，而不需要單獨的 issue 時，即表示完成。

由索引模型根據 Issue 內容生成。