Implement \eyeglasses as markup command

Implement \eyeglasses as markup command
-) implement \eyeglasses as markup command, using the postscript stored
   in the eyeglassesps variable
-) Use this markup command in musicxml2ly, too
-) Add regtest for \eyeglasses (in a \markup and as a markup attached to
   a note)

[joeneeman, 2009-02-20 18:56:08]

Could you add the command to appendix B.8.3 of the manual? And maybe change the
example for the \postscript command. Otherwise, lgtm