Add retry_queue to sidekiq_options

```ruby
sidekiq_options queue: 'high', retry_queue: 'low'
```

Allows failed jobs to be retried on a new queue (so their priority can be adjusted relative to new jobs of the same class)

Would be happy to implement a ``retry_options`` with more options or to do so in separate middleware if you think this is too specific to be included in core sidekiq

P.S. thanks for the gem, it's awesome