Add support for specifying custom RQ `Worker` and `Queue` classes.
New configuration options:
CLI Argument | Env Variable | Default Value | Description
------------ | ------------ | ------------- | -----------
`--worker-class` | `RQ_WORKER_CLASS` | `rq.Worker` | RQ worker class
`--queue-class` | `RQ_QUEUE_CLASS` | `rq.Queue` | RQ queue class