Fix support for :opts key in AMQP pubsub client broker configuration

[iarenaza]

README.md documents that there is an optional :opts key in the broker configuration that can be used to pass additional AMQP connection options to the underlying library. But the whole :opts key (and associated value) is passed in as another AMQP connection parameter. Which means the underlying AMQP library completely ignores it (as it doesn't use such a key at all).