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: A La Mode De France
T: Nonesuch
N: Change: RuftyTufty.abc
M: C
L: 1/4
Q: 160
K: Gmaj
P: A
"D" d | "D" d B c d | "G" B>A G d | "G" d B c d | "G" B3 :|
P: B
|: "G" B | "D" A F G A | "G" B>A G B | "D" A F G A | "G" B3 B |
"D" A F G A | "G" B>A G d | "D" d B c d | "G" B3 :|