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
```

Consider moving `rqworker` and `rqinfo` into the same `rq` script, so we only need to distribute a single top-level script. Maybe we can ship the `rqworker` and `rqinfo` scripts for backward compatibilty reasons, but we should print a notice on stderr that the new way of running these scripts is to use `rq info`.