added beforeTouchDown

[noahpin]

beforeTouchDown functions similarly to beforeMouseDown, in that it allows for disabling or filtering zooming/panning via touch. Ex:

panzoom(element, {
  beforeTouchDown: function(e) {
    // allow touch-down panning only if there is more than one touch active
    var shouldIgnore = !(e.touches.length > 1);
    return shouldIgnore;
  }
});