An option to set an un-rendered time signature

[newlandsvalley]

See https://groups.google.com/forum/#!topic/vexflow/Pw9gPahvc40.

Mohit says there's a slight bug in the spec:

Right now if you don't specify a time signature, it simply assumes 4/4.
If you do, it always renders it. 
What we need is an option to set the time signature, but not render it.