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: Sonny Brogan's Mazurka
M: 3/4
L: 1/8
R: mazurka
K: Dmaj
|: FG | A2 AG FA | d2>A2 BA | G2 GB AG |
| FG EF DF | A2 AG FA | d2>A2 BA | G2 Gg fe | d4 :|
|: e2 | f2 fa gf | e2 eg fe | d2 df ed | cd Bc A2 |
| f2 fa gf| e2 eg fe | dc df ec | d4 :|