* Remove requirement for synchronous rules to accept, and
invoke, a `done` handler;
* Add support for returning promises, generators, from
rules.
* Add `doc/external.md`, with a description on how to use
and create external rule packages;
* Add structural fixtures per-rule inline, which are now
shown in (improved) `doc/rules.md`, and used in testing
each rule;
* Add contributors to `package.json`;
* Remove history before stable from changelog;
* Refactor tests to use `tape` instead of `mocha`;
* Add automated generation of `rules.js` index;
* Fix incorrect badges.
