function getValue(start, end, elapsed, duration) {
  if (elapsed > duration) return end;
  return start + (end - start) * easing(elapsed / duration);
}
Given a start/end point of a scroll and time elapsed, calculate the scroll position we should be at
@param {Number} start - the initial value
@param {Number} stop - the final desired value
@param {Number} elapsed - the amount of time elapsed since we started animating
@param {Number} - duration - the duration of the animation
@return {Number} - The value we should use on the next tick