Implement rule-delimiter (#210)

See #210.

@mrpavlikov, @kosteQ, @ognia, I've rebased [#302](https://github.com/csscomb/csscomb.js/pull/302) for you.
To push further changes you can either send PR's with commits to `mrpavlikov/210` branch or fetch this branch to your repo and open another PR to `dev` branch.
Why is `unknown` in this group?