New prop for disabling draggable

[chrisjpatty]

I've found in my use-case scenarios that it would be nice to pass a boolean prop to the <Draggable/> that could disable it. This is especially useful in cases where a child of the Draggable also needs to be able to handle click or tap events.

[sisp]

I'm having a similar case with nested draggables/droppables. My workaround is to inject event.stopPropagation() into onMouseDown/onTouchStart callbacks of <Draggable /> and into onMouseEnter/onMouseLeave callbacks of <Droppable />.