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.

X: 1
T: Shooting
T: Beaux of London City
A: Adderbury
M: 9/8
L: 1/8
P: 6(2A.3B)
K: G
P: A
"" D | "G" G2 G BAG B<dD | GDG B2 "D" A A2 D |\
       "G" G2 G BAG B<dD | GDG B2 "D" A "G" G3 :|
P: B
"G" d2 c B>AG B/d-d | "D" A2 A A2 "G" B "D" c3 |\
"G" d2 c B>AG B/ d2 | GDG B2 "D" A "G" G3 :|