Support promises on callback-before

[xaralis]

It would be nice if callback-before would support thenable objects (promises) as well. It would simplify things when one needs to trigger the scrolling after some other promise (which is triggered by clicking the link) is finished.

For example, we have overflow: hidden on document when showing an aside menu. When user clicks a menu item, it is closed returning a promise and then we would like to scroll to proper element on the page. However, since closing the aside menu takes some time, scrolling doesn't work most of the time, because it does not wait for callback-before to finish in case it's an async call like promise.

Hope it makes sense. I think that the change will be very simple. Can post a PR if interested. It just seems that callback-before might not be the right place for this.