Add --contains flag to abcfield.py. Currently we rely on any N: field with a Change: subnotice being the first N: field. Add --contains field to abcfield.py to remove this restriction. Also use it to extract new subfield Credit: and add that if present to the tune information in print and on web.

This book is for tunes that may not be part of the regular Havoc session or dance repertoire, but which I want to try over.