Replace csslint with stylelint and better control

http://stylelint.io is a modern CSS linter which shared a lot of commonality with eslint. It's got a growing community with Wordpress, Facebook, Wikimedia and GitHub adopting it.

Nice overview of stylelint and why you would want to do it: https://css-tricks.com/stylelint/

This issue is about defining rules and linting from grunt. We can do this for plain css and less now, but it is flexible to support scss or whatever comes next too.