• Added arbitrary boundary selector. Now you don't have to just use 'parent', you can select any element
  on the page, including 'body'.
• Bugfix: Prevent invariant if a <Draggable> is unmounted while dragging.
• Bugfix: Fix #133, where items would eagerly start dragging off the mouse cursor if you hit boundaries and
  came back. This is due to how <DraggableCore> handles deltas only and does not keep state. Added new state
  properties slackX and slackY to <Draggable> to handle this and restore pre-v1 behavior.