Allow multiple \with per context creation

Allow multiple \with per context creation

This allows using more than one \with context modification in
connection with \new, \context, \addlyrics, \drums, \lyrics, \chords,
\figures .  While combining them inside of a single \with {...} is
possible, sometimes it may be inconvenient.

[Dan Eble, 2016-06-18 12:42:19]

LGTM

https://codereview.appspot.com/296470043/diff/1/lily/parser.yy
File lily/parser.yy (right):

https://codereview.appspot.com/296470043/diff/1/lily/parser.yy#newcode1497
lily/parser.yy:1497: context_modification_mods_list:
Comments as simple as these would help a person find his bearings in this file:

/* a series of \with ... \with ... */
context_modification_mods_list

/* the statements inside a \with block */
context_mod_list

More distinctive names would be preferred.  (I know that's hard.)  Maybe the
"modification_mods_list" could be "modification_group_list" or something like
that.

[dak, 2016-06-19 11:02:43]

Added some comments.  Dan's proposed names were not following any inner logic,
so not taken.