Adding support for isEqual completion handler

How hard would it be to add a completion handler for the diff function so the values don't have to be equatable? A library called Diff does this. 

For example: 

```
        [1,2,3].diff([3,4,5]) { (old, new) -> Bool in
            return old == new
        }
```

This would allow me to pass an array of custom models and only check if certain values are equal. 

Thanks,
Joe
