Expose methods for unsubscribing and getting a channel subscription object.
Expose methods for unsubscribing and getting a channel subscription object.

Store subscription objects on an internal object and expose
three new methods: `subscription(channel)`, `unsubscribe(channel)` and
`unsubscribeAll()`

amritsingh/private_pub_plus,PeritusSolutions/peritus_private_pub,czhu12/private_pub_sharded,teelu/private_pub,clientelle/private_pub,loomio/private_pub,SolarCS/private_pub,ovinix/private_pub,Bunlong/private_pub,twetzel/private_pub,milushov/private_pub_client,Molinos/private_pub,ovinix/private_pub,he9lin/private_pub,Bunlong/private_pub,Molinos/private_pub_client,czhu12/private_pub_sharded,czhu12/private_pub_sharded,larsharzem/private_pub,milushov/private_pub_client,clientelle/private_pub,loomio/private_pub,ryanb/private_pub,larsharzem/private_pub,twetzel/private_pub,PeritusSolutions/peritus_private_pub,ryanb/private_pub,Molinos/private_pub,Molinos/private_pub_client,he9lin/private_pub,SolarCS/private_pub,teelu/private_pub