Added the support for vertical swipe direction.

Hey,

I just found out your lib, thanks for the sharing, this is really well done.
I had the need for the support of the vertical swipe, so I added a "allowedSwipeDirections" property in the MDCSwipeOptions. By default it is set to MDCSwipeDirectionLeft | MDCSwipeDirectionRight. I had to change the const value of the MDCSwipeDirection in order to use the "OR" operator properly.