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: Winster Gallop, The
M: C
N: Change: SalmonTails.abc
L: 1/4
Q: 240
R: polka
K: Gmaj
"G" GB GB | GB d2 | "D7" A>c BA | "G" GB d2 |
"C" c e g e | "G" d B d2 | "D7" AB/c/ BA | "G" G2 G2 :|
|: "G" {c/d/} gf ed | gf ed | gf ed | "D7" cB A2 |
"D7" fe d2 | fe d2 | d2 A>c | "G" B G G2 :|