Add per-file rule setting a la JSHint

[webbower]

It would be awesome to be able to manually enable/disable Lint rules per file similar to how JSHint does it with a specially formatted comment at the top of the file.

[miohtama]

Yes. Curretly csslint cannot be enforced 100%, because you have valid use cases which are exceptions to rules. For example, if you use it with a text editor with background linting support like Sublime Text 2, you get unnecessary clutter in UI due to warnings which are not actually warnings but actual CSS code you wish to implement.

[frvge]

See #564.