Change options of the `rq empty` command.

* Does not ask confirmation
* Does not need `--yes` option anymore
* Added --all, to clear all queues
* No implicit emptying of the failed queue.  If you want to empty the
  failed queue, just use `$ rq empty failed`

rq/scripts/rq_cli.py

@@ -27,33 +27,23 @@ def main(ctx, url):
 
 
 @main.command()
-@click.option('--yes', '-y', is_flag=True, help='Empty failed queue by default')
+@click.option('--all', '-a', is_flag=True, help='Empty all queues')
 @click.argument('queues', nargs=-1)
 @click.pass_context
-def empty(ctx, yes, queues):
-    """[QUEUES]: queues to empty
-
-    \b
-    $ rq empty
-    Do you want to empty failed queue? [y/N]: y
-    2 jobs removed from failed queue
-    \b
-    $ rq empty -y
-    2 jobs removed from failed queue
-    \b
-    $ rq empty default high
-    10 jobs removed from default queue
-    2 jobs removed from high queue
-    """
+def empty(ctx, all, queues):
+    """Empty given queues."""
     conn = ctx.obj['connection']
-    queues = [Queue(queue, connection=conn) for queue in queues]
+    if all:
+        queues = Queue.all(connection=conn)
+    else:
+        queues = [Queue(queue, connection=conn) for queue in queues]
+
     if not queues:
-        if yes or click.confirm('Do you want to empty failed queue?', abort=True):
-            queues = (get_failed_queue(connection=conn),)
+        click.echo('Nothing to do')
+
     for queue in queues:
         num_jobs = queue.empty()
-        click.echo('{0} jobs removed from {1} queue'.format(
-            num_jobs, queue.name))
+        click.echo('{0} jobs removed from {1} queue'.format(num_jobs, queue.name))
 
 
 @main.command()