Support built-in CSSLint formatters

[lazd]

csslint.reporter() should take a string indicating a built-in CSSLint formatter ala gulp-jslint.

CSSLint formatters: https://github.com/stubbornella/csslint/blob/master/src/formatters/text.js

[lazd]

@SimenB I think closing this would get us to 1.0. Current reporters are totally custom and not related to CSSLint reporters, which is bad. I haven't thought about the issue in a while and I'm not sure what the best way to handle it is, but certainly working with CSSLint's formatters is required.