Refactor Gren.js

[Gren.js](https://github.com/github-tools/github-release-notes/blob/master/lib/src/Gren.js) is already have 975 lines and IMO it's very difficult to maintain it.
I think it should be split to a couple of modules: release, changelog, issues, commit, milestones, prs.

@alexcanessa what do you think?

Start by reading lib/src/Gren.js, which the issue identifies as the large module to refactor. Map its responsibilities against the proposed release, changelog, issues, commit, milestones, and prs modules. Done means those responsibilities are separated into maintainable modules without changing release-note behavior; the issue does not mention tests to run.

索引モデルが issue の本文から書いたものです。