Callback function

Adding a callback function option to settings. If set, the function is executed once the bar is fully loaded. This can be useful as NProgress.done not always might be used in order to indicate the completion of the bar load. For instance, I recently used NProgress in a project where NProgress.set(percetageStep) was called in many different places. So instead of checking the bar progress status in every of those places, using a callback function simplified my code.