Support JSON for .csslintrc files.

@stubbornella [requested a PR for this](https://github.com/stubbornella/csslint/issues/359#issuecomment-35819236) 2 days ago. I [provided this change](https://github.com/stubbornella/csslint/issues/359#issuecomment-15096096) 11 months ago, so it may be stale. I honestly haven't tested since then. Hopefully someone else involved in the discussion (@tkellen, @cowboy, @jzaefferer) can check if this still works and how it'd integrate with grunt-contrib-csslint.