Comments for DDG::Goodie::ABC

This is a fix for #342.  The goal was to add comments to DDG::Goodie::ABC.  I got carried away perhaps and did some refactoring.  Let me know if I've gone too far.  

The reason I ended up doing some refactoring was the code was trying to be flexible and allow multiple word choices but at the same time it was not allowing multiple word choices.  I hopefully simplified things by doubling down on not allowing multiple word choices.