Explicit Prev/Next Button Callbacks

Added explicit callbacks for previous and next buttons, so unique functions can be called for each button.

```js
var elem = document.getElementById('mySwipe');
window.mySwipe = Swipe(elem, {
  startSlide: 4,
  auto: 3000,
  continuous: true,
  disableScroll: true,
  stopPropagation: true,
  prevClicked: function() { /* called when previous button clicked */ },
  nextClicked: function() { /* called when next button clicked */ },
  callback: function(index, element) {},
  transitionEnd: function(index, element) {}
});
```