Run CSSLint as part of codechecker checks

https://github.com/CSSLint/csslint is available and fortunately provides a checkstyle output, so we can integrate it nicely like we do with jshint.

Problems:

1. The rules can be very subjective e.g. https://2002-2012.mattwilcox.net/archive/entry/id/1054/
2. We don't have any CSS guidelines
3. With less it is even relevant

That said, even if we just had the most basic 'is it broken' thing as an error and then released more and more things as errors, it would be a nice step improvement.