This pull request adds callbacks for when the user starts or stops dragging the slider. The start drag event passes the starting frame, while the end drag event passes the end frame.
Additionally it adds a second optional parameter to the play method for playback direction. This allows a user to change playback direction on the fly.
A use case for these enhancements is stoping playback while a user is dragging, then resuming playback in the dragged direction after the user stops.
Added callbacks onDragStart and onDragStop
Added direction as an optional argument to the play method