Collection.add accepts negative indexes for the `at` option

https://github.com/jashkenas/backbone/pull/3281 added support to `at` for negative indexes but it seems useful to add to `add` as well. Adding a model to the end of a collection would be as easy as:
```JS
collection.add(model, {at: -1});
```

With the simplicity of the change and since passing -1 would add to the beginning of the collection (the opposite of the user's expectation) without this fix I think it's worth accepting.