Exclusive Subsets

Hey,

I ran into an issue with exclusive subsets; because, I wasn't sending options when adding/removing models. So when the code went to extend the options with {exclusive_collection: true} it would crash.

This code checks for undefined options. It also let's you define this.exclusive_collection as a Boolean instead of having to pass a function. Function still works, but now it's just a little less code to use.

Thanks for the awesome lib! Saved me a ton of work.

- Jordan