Allow ability to scroll screen on drag

Currently, if dragging brings you to the edge of the viewport, the list will not scroll down. Ideally, it would be nice to have a feature similar to the one in jQuery UI. Source.

This can be tested simply on the dragula home page, put the page like this:

Grab an object and drag it to the bottom, the list will not scroll. This makes dragula unusable for long lists on smaller screens.

Thanks!