Add line numbers and update tests

[mduvall]

Augmented the RuleSet struct to contain the metadata about the line number, would be useful to help implement #39 and pass the data through redundancy_analyzer.rb.

[zmoazeni]

Nice! I have some issue with the implementation, but you definitely figured out the hard part. I forgot parslet returned a "slice".

[zmoazeni]

@mduvall I think I'm going to take this one, but I appreciate the PR. We have a file mapping issue in the case where a user runs csscss file1.css file2.css and I don't know how much I love the Metadata stuff. I think we can push this behavior into the class itself and then only deal with it in the tests when we care.

But you certainly hit the nail on the head by using Slice#line_and_column, that was the hard part about this functionality that was eluding me. The rest should be gravy. /cc @kschiess

[mduvall]

@zmoazeni Agreed, after some thought it seemed like it would make more sense for the data to be with the declaration, especially since the declaration is specifically on it's own line in most CSS files - thanks for the feedback again :)

[zmoazeni]

I'm going to close this unmerged. But I'm planning on spending some time to implement this.