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: 15
T:Lanigan's Ball
R:Jig
H:Seamus Quinn
M:6/8
L:1/8
Q:3/8=120
K:G
|: "Em" E2E G2A|B2A Bcd| "D" D2D F2G|ABA AFD|
"Em" E2E G2A|B2A Bcd|"Bm" eBB B2A| "Em" BEE E3:|
"Em" e2f g2e|fag fed| "D" e2f gfe|fdB Bcd|
"Em" e2f g2e|fag fed| "Bm" edB cBA|[1 "Em" BGE EDE:|[2 "Em" BGE E3|]