[Carousel]: Add configurable swipe threshold #941

Angular's Carousel now has a configurable swipeThreshold (PR #733). React currently snaps slow drags to the nearest position, effectively using a 0.5 threshold, with no way to change it.

Add a swipeThreshold prop to Carousel.Content for drag release snapping. It should work with looping and bounded carousels, including a shorter fractional final step, while preserving quick flick behavior. Document the prop and test the default and custom values. Decide whether the default should retain React's current behavior or match Angular's 0.3.