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.

syntax: glob

*~
*.ps
*.pdf
*.aux
*.log
*.orig

syntax: regexp
build/
graphics/
web/Morris.*/
web/Session.*/
Morris-.*
Session-.*