Variadic PING with support for Pub/Sub.
Variadic PING with support for Pub/Sub.

PING can now be called with an additional arugment, behaving exactly
like the ECHO command. PING can now also be called in Pub/Sub mode (with
one more more subscriptions to channels / patterns) in order to trigger
the delivery of an asynchronous pong message with the optional payload.

This fixes issue #420.

JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis