adding the ability to know if there are requires that can't be parsed

Solves this issue: https://github.com/pahen/madge/issues/14 . Note that this is a breaking change, since this requires another set of results to be added to the return value of madge parsing. I need to use this on a project I'm working on, so I couldn't wait for comment, but please give me feedback and I'll tweak it as needed.