Add stylelint engine for .css linting

[Iain-S]

Issues with CSSLint

Running CSSLint via codeclimate has several issues:

1. Most importantly, I can't get it to pay attention to .csslintrc so we have no way to ignore rules (I'm pretty sure it also ignores commandline args when run via codeclimate so we can't do --ignore="rule1","rule2").
2. CSSLint isn't maintained so many of the rule violations are actually CSSLint bugs.
3. When run via codeclimate, it doesn't print the name of the broken rule, only the offending file and linenumber.
4. If you install csslint with npm, the default formatting still doesn't print the rule name. I had to do csslint --format=compact public/css/ | sed 's/line/\n\nline/g' to get a useful output. The standalone csslint does at least adhere to .csslintrc.

All in all, I'm not sure the codeclimate csslint plugin is that useful as a CI tool. Though you may still want to use it to find the * and *= issues (though even this is disputed).

stylelint

Stylelint doesn't pick up all of the same issues as CSSLint but

1. The VSCode plugin is nice.
2. The standalone stylelint comandline tool has a --fix option to automatically fix some issues.
3. When run with codeclimate -e, stylelint does print the name of the broken rule(s).

[codeclimate]

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[yeban]

Thanks!