Enhancement: Implement parenthesizing spanners

[Simon Albrecht]

Lily is being very honest and makes a kind warning telling us that she 
doesn’t know how to parenthesize spanners. But it would be really nice 
if we could teach her that :-)

%%%%%%%%%%%%%%%%%%%%%%%%%%
\relative { c''-\parenthesize \> c c c
  c2\! -\parenthesize ~ c
}
%%%%%%%%%%%%%%%%%%%%%%%%%%

<https://code.google.com/p/lilypond/issues/detail?id=4565>