Feature Request - Iteratee for difference / union / without

[hunterSThompson]

It would be cool to be able to override default with an iteratee in difference, set, and the other set operations in Underscore.

For example:

var obj1 = {id: 1, name: "John"};
var obj2 = {id: 2, name: "Bob"};
var obj3 = {id: 3, name: "Steve"};

var list1 = [obj1, obj2, obj3];
var list2 = [obj1, obj2];

var difference = _.difference(list1, list2, function(x, y) { return x.Id == y.Id });
// difference -> [obj3]

Would this be useful to anyone else? Can start working on a patch if so.

[akre54]

This seems like it'd be useful. Send that patch!

[jdalton]

There's a lot of discussion on #2311 around this kind of thing. Dup of #2311.

[binarykitchen]

Me too would welcome this feature. Looking at the long thread in #2311 doesnt help. Can't someone summarize the solution here?