Allow group options to have a function for pull

[rockhymas]

Fixes #504

@David-Desmaisons I saw your comments in the related issues. I believe this deals with them properly, but if not, I'm happy to keep working on this until it's correct. Just point me in the right direction.

[rockhymas]

Ping

[David-Desmaisons]

Thanks for the PR.
I am still not convinced of the interest of using a function for pull since the option prop is reactive. Moreover passing DOM element to the function does not sound to me like a gooe pattern.
That said, the cloning mode had to be cached on the instance as you are doing.
I should provide this feature in the short term.

[JivanRoquet]

Hi there,

Just my two cents concerning the interesting of using a function for pull.

@David-Desmaisons is there any other way you would think of, when trying to make the pull behaviour reactive to the ctrl/cmd buttons, so that:

• dragging with Ctrl/Cmd pressed => duplicate the item (leave it in place in original position, and create a new one)
• dragging without Ctrl/Cmd pressed => move the item as usual

I've been trying to do that with pull as a function, but no luck so far. Any other way to get to the same behaviour would be massively welcome.

If using pull as a function is the only way to get to that, then I think that this would be indeed a very useful thing to do, given that being able to alter the move/copy behaviour with Ctrl/Cmd keys while dragging is quite a standard and very widespread thing.

[David-Desmaisons]

@JivanRoquet I understand and I agree with your arguments.
Pull as function will be provided.
I made a PR to Sortable.js to get all the information to handle it correctly in vue.draggable.

[rockhymas]

Just wanted to chime in that I've been following the work and am glad it's going forward, thanks @David-Desmaisons!

[David-Desmaisons]

Fixed by PR #584