Enable the use of parameters in scopes

By passing in the controller it self to the scopes when we evaluate them, we can do stuff like checking controller parameters to check what we should filter with the scope.
So, there is another way to approach this whole thing, by using a default method that can be overridden if wanted. It's definitely less hacky, doesn't have any `instance_eval`/`instance_exec`. Something like this:

```ruby
def index
  @items = Item.order(sticky: :desc)

  @items = custom_scopes(@items)

  @items = @items.kaminari_page(params[:page] || 1)
  @items = @items.per(params[:limit] || 25)

  respond_with @items
end

protected

def custom_scopes(items)
  items
end
```

In the wrapper app, you'd just make your own ItemsController, include the concern and override _custom_scopes_ method.