Implement pub/sub convenience commands

[sebastienfilion]

I created a separate ticket for the pub/sub commands because SUBSCRIBE and PSUBSCRIBE are blocking commands.
My high level idea for these commands is that they would an async iterator that could be passed to stream.

Although PUBLISH, UNSUBSCRIBE, PUNSUBSCRIBE and, PUBSUB can be factories like all the other commands, subscribe would be better suited as a function of the client submodule.