Add tag explain

[Qix-]

After #27, tag explain will show a highlighted synopsis given a particular context/set of tags or variables in the terminal that shows why a particular line/rule/dependency/etc is being included/excluded given that context.

Purely a debugging tool - it'll (most likely) require a color terminal, unless someone can come up with a better way to show this information (similar to diff output - in which case, I'd only feel comfortable with it if there was a way to parse that output already available on npm prior to releasing that functionality, for the sake of adoption).