Revert event callback function

[mavrosxristoforos]

Hi there and thank you for creating this plugin! It works very well.
I needed a revert event callback for Draggables, so I added an else block for the if ($droptarget) { in the dragstop event of the Draggable that does a simple check for a self.options.onrevert and triggers it. If you think it's useful, I could possibly include it in a PR.

[gardiner]

Hi Christopher, thanks for your message! If you do find the time and create a pull request, I will take a look and try to include it. Cheers! Ole.

[mavrosxristoforos]

Thanks! I have created the pull request as suggested. Please revise whenever you can.

[gardiner]

Thank you!