ENH: add subok flag to stride_tricks (and thus broadcast_arrays)

As is, `np.broadcast_arrays` does not preserve subclasses, even though for the shape manipulation subclasses should not be a problem. This PR adds a `subok` option that allows one to change that, but is `False` by default, so that existing code will continue to behave as is.
And yeah, you are right, it deals with a seperate issue... I somewhat thought it might make this one simpler, too, but that doesn't change much.