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: Jamie Allen
M: C
L: 1/4
Q: 160
K: Gmaj
|: G/A/ | "G" B G G A | B2 G A/B/ | "D7" c A A B | c2 B A |
"G" G g "C" g e | "G" d2 B B/c/ | "D7" d d c/B/ A | "G" G3 :|
|: B/A/ | "G" G g g f | e d c B | "D7" A a a g | f d e f |
"G" g>a "C" g e | "G" d2 B B/c/ | "D7" d d c/B/ A | "G" G3 :|