add: subscribe_uniq/3

[mad42]

I've recently encountered edge cases when I subscribe to an already subscribed topic, multiplying received events.
subscribe_uniq/3 prevents this from happening by looking up in the Registry for pubsub, topic before registering it.

[chrismccord]

This will only allow a single subscriber for any process, so this is not what you want. Note the docs on duplicate subscriptions:
https://github.com/phoenixframework/phoenix_pubsub/blob/master/lib/phoenix/pubsub.ex#L98-L104

The best solution is to avoid duplicating the subscribe in the caller, such as checking your state. Thanks!