[Behavior] Maintain a changelog

[benbalter]

When a pull request is merged, add a line to CHANGELOG.md or HISTORY.md, with the PR title, number, and if the contributor is not a maintainer, give them props, optionally categorizing the updates based on tags.

@jekyllbot does this (with a slightly different behavior that I don't like), but the end result could look somethign like this:

## Head

### Minor

* Make the button red (#123, props @bkeepers)

### Major

* Button is now a switch (#456)

### Development

* Add Rubocop (#789)

[bkeepers]

After #90, this should be implemented as a separate plugin. Please do investigate.