Update URL before animation

Instead of waiting for the animation to stop before updating the URL, let’s update it right off the bat.

I’m using `nav:target` to show/hide my main menu on touch devices using only CSS. One of the benefits of this is that as soon as a menu item is tapped, the menu is hidden because the URL changes. Enter smooth-scroll: Because the script waits until the animation stops before updating the URL, the menu stays visible while the script animates the scroll to the selected location, and **only then** closes. This looks awkward.

This commit changes the order of things, so that the URL is changed immediately, making my menu toggle at the expected time (immediately after tapping a menu item).

Let me know if you would prefer a more robust solution that offers both options. For example: dataURL could have the values `before`, `after`, and `false`, so developers have more control over this behavior. I’d like to take a crack at that if it’s something you’d like to see.

Thanks!