Add mouse drag/fling support.

Hey @petehunt,

I’m exploring adding other input method support to this as is. I have a few concerns on my implementation I’d appreciate your feedback on.
This bit right here is namely where my concern is. I suspect that the `ResponderEventsPlugin` may be what I need to keep `mousemove` events focused on initial target to match the implementation of `touchmove` events. Is this correct?