Let parser accept symbols after \new, \context, \unset and implicit \set (issue 13180044)

[janek . lilypond]

Looks like a good thing to do, but what does this change mean?  Will we
be a) able to do things that were previously impossible or b) is there
no user-visible difference?
I think the answer is b), but i'd like to make sure.

https://codereview.appspot.com/13180044/