[Request] NACK with a delay

Hi, I'd like to implement [retry with an exponential backoff as used in Sidekiq,](https://github.com/mperham/sidekiq/wiki/Error-Handling#automatic-job-retry) that is if a job fails it is requeued with an ever longer pause.

Right now if I NACK a job, it returns to the queue immediately. My other option is to ACK the job and add it again, with a delay. This works OK, but from client's point of view it changes the job ID and that makes tracking failed jobs more difficult.

The NACK command with a delay support could look like this:

`NACK ... [DELAY ]`

What do you think?