changelog: add script to generate HTML from CHANGES.md

Rationale:

  - CHANGES is a useful file to include in the repository.

  - Markdown files are nice to read in a terminal; HTML files less so.

  - Markdown is more writer-friendly, too. (@jashkenas raised the issue of ease of authoring in #1457.)

  - "Diff" and "Docs" links are automatically inserted, removing a source of human error.

  - By introducing a template, future markup changes need only be made in one place.

If there's support for this approach I'll complete [CHANGES.md][1] and tweak the CSS to avoid introducing visible changes.


[1]: https://github.com/davidchambers/underscore/blob/generate-changelog/CHANGES.md