Add quiver.strings.center

I haven't seen `rightFill` on any other `center` implementations, but I thought it would be useful for things like brackets, e.g. '<<<<<Some Text>>>>>'.  I can remove it if it seems gratuitous.  Could also make it a named parameter.

Padding the extra char on the right for odd pad counts was taken from ruby.  Python does the opposite.  Can switch if desired.  Or could add a `preferRight` flag or something, but most use cases probably don't care.