Adds Flag to Fingering side support list (issue 6201079)

[k-ohara5a5a]

http://codereview.appspot.com/6201079/diff/1/input/regression/fingering-flag.ly
File input/regression/fingering-flag.ly (right):

http://codereview.appspot.com/6201079/diff/1/input/regression/fingering-flag.ly#newcode4
input/regression/fingering-flag.ly:4: texidoc = "Horizontal fingerings
avoid flags.
add to 'finger-chords.ly' instead, so we get a test-case for each choice
of add-stem-support

http://codereview.appspot.com/6201079/diff/1/scm/define-grobs.scm
File scm/define-grobs.scm (right):

http://codereview.appspot.com/6201079/diff/1/scm/define-grobs.scm#newcode875
scm/define-grobs.scm:875: (add-stem-support . #t)
We need some docs changes if the default changes

http://codereview.appspot.com/6201079/