Make stats command be subcommand

This makes the stats command be a sub command of container.
It also makes the id provided via the first arg instead of a flag.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

ctr/main.go

@@ -37,7 +37,6 @@ func main() {
 		CheckpointCommand,
 		ContainersCommand,
 		EventsCommand,
-		StatsCommand,
 	}
 	app.Before = func(context *cli.Context) error {
 		if context.GlobalBool("debug") {