PUBSUB command implemented.

Currently it implements three subcommands:

PUBSUB CHANNELS [<pattern>]    List channels with non-zero subscribers.
PUBSUB NUMSUB [channel_1 ...]  List number of subscribers for channels.
PUBSUB NUMPAT                  Return number of subscribed patterns.
Allow PUBSUB NUMSUB without channels.

The result is an empty list but it is handy to call it programmatically.