added default_models_query

```ruby
class UserManagerWorker < Sidekiq::ActiveRecord::ManagerWorker
  sidekiq_delegate_task_to UserTaskWorker
  default_models_query -> { User.active }
end

UserManagerWorker.perform_async(:batch_size => 300)
```

this is a solution to: https://github.com/sidekiq-orm/sidekiq-activerecord/issues/3
@seuros /cc