'direction' option

Hi,

Currently the library invokes on every swipe/drag event. Including up,down,left and right. In my project the swipeleft event is already used for navigation purposes. And being able to invoke a delete button by dragging up and down just doesn't feel right to me.

So that's where the direction option will be very usefull for me. I can just pass the desired direction and it works :). If nothing set, it has the old behavior.

I have added to the demo an example.