making droppablesActive as a config

[nikhilkataria-salesforce]

This solves the perf issue discussed in #39.

One can now set isDroppablesActive false as part of DragOptions while making the source as follows:
jsPlumbInstance.makeSource(idList, dragOptions: {
isDroppablesActive : false
})

[sporritt]

Thanks for this. But I'm not sure I want to merge it. In #39 I mentioned a different possible fix which would be preferable from my point of view because it doesn't introduce extra complexity.

If we added this functionality it would of course be off by default. Then to know it has to be switched on a user first has to encounter the issue you encountered, then take it upon themselves to investigate, which ought to mean reading the documentation but in practice I can assure you it means asking me. So I would write some more docs that I'd have to maintain and noone would read, and there'd be more things for people to know, levers to pull, etc.