Scroll while dragging implementation

Hi, I'm using slip as part of my work and amongst other things, we needed the scroll-while-dragging-near-the-edges behavior… so here's a PR for that :). Thanks for the great work so far!


BTW, "Slip" in french means "Underpants". I had a fun time asking my boss if it was ok for me to contribute back to a project called Underpants :D