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: Mazurka d'Avignon
M:3/4
L:1/8
Q:120
K:G
"G" DG GF G2 | DB BA B2 | "Am" cB AG FG | "D" AG FE D2 |
"G" DG GF G2 | DB BA B2 | "Am" cB AG FG | "D" AB "G" G2 z2 :|
(Bc) |\
"G" d2 de dc | B2 B2 AB | "C" c2 cd cB | "D" A2 A2 (Bc) |
"G" d2 de dc | B2 B2 AB | "Am" cB AG FG | "D" AB "G" G2 z2 :|