Add CLI `rq` to empty queues and requeue failed jobs

```bash
Usage: rq [OPTIONS] COMMAND [ARGS]...

Options:
  -u, --url TEXT  URL describing Redis connection details.
  --help          Show this message and exit.

Commands:
  empty    Empty queues, default: empty failed queue $...
  requeue  Requeue all failed jobs in failed queue
```
e.g.
```bash
$ rq empty
2 jobs removed from failed queue

$ rq -u 'redis://localhost:6379/0' empty default high
10 jobs removed from default queue
2 jobs removed from high queue

$ rq requeue
Requeuing 4 failed jobs......
Requeue over with 0 jobs requeuing failed
```

The `empty` command shouldn't default to failed queue, it's too easy to do this by accident. We should also display a confirmation before deleting anything unless a -y flag is given.