Allow custom easing function

[ciromattia]

See bfintal#35

E.g. (thanks to @weisk)

...
var step = 0;
var easing = 'easeInOutQuint';
var f = function () {
  step++;
  var progress = step / divisions
  var delay = easings[easing](progress);
  delay = parseFloat(delay.toFixed(2));
  delay = delay * 20; // tends to 20 ms
  ...
  if ($this.data('counterup-nums').length) {
    setTimeout($this.data('counterup-func'), delay);
  }
...