Abstract review file contents

[danobi]

Right now we pass around strings and string references everywhere. It mostly works, except in a few places it's ambiguous whether or not text is quoted or unquoted text that represents quoted text (or some other combination). It would be great to unify all text behind a data structure ike LineType:

https://github.com/danobi/prr/blob/6b4a4bf5268a7c8841a063c285fccff979ca6633/src/review.rs#L51-L59

That way, we can unambiguously pass around Vec<LineType> (or similar) and unify all the quoting / dequoting.