Use markdown reference-style links for github usernames in CHANGELOG
Use markdown reference-style links for github usernames in CHANGELOG

Inspired by [Rubocop's CHANGELOG](https://github.com/bbatsov/rubocop/blob/master/CHANGELOG.md).

whitesmith/rubycritic,whitesmith/rubycritic,whitesmith/rubycritic,whitesmith/rubycritic