Allow custom log handlers

add `use_default_log_handler` option to `work` function, default is
True, it means use rq default logging settings.

sometimes i want use custom logging settings. so i can disable rq
default logging. like:

```
def main():
    # some argparse code ...
    args = parser.parse_args()
    setup_logging(args.config)

    # some preload codes ...

    with Connection(env.Redis):
        queues = list(map(Queue, env.listen_list))
        worker = Worker(queues)
        worker.work(use_default_log_handler=False)

if __name__ == '__main__':
    main()
```