Events for all flipsnap states with preventable slide transition event

Hi! This patch lets you catch events on the touchstart of the flipsnap, the touchmove of the flipsnap, and the touchend of the flipsnap. You can also prevent the touchmove slide event in order to e.g. override specific behaviour for a particular frame:

	// override backward transitions to happen instantly (no smooth slide)
	flipsnap.element.addEventListener('fstouchmove', function(ev) {
		if (ev.direction < 0) {
			console.log("bckwrds");
			ev.preventDefault();
			ev.stopPropagation();
			flipsnap.toPrev(0);
			return false;
		}
	}, false);

Thanks!