Add `subscription` callback to  PrivatePub.sign
Add `subscription` callback to  PrivatePub.sign

If options given to `sign` include a `subscription` function, it
will be called with the newly created subscription object. This
way users can add a Faye callback to know when a subscription is
ready or cancel a subscription in the future.

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