Allow specifying an optional queue namespace

It would be nice to be able to specify a "namespace" for the queue to avoid conflicts and make it easy to identify what the queue is for e.g. prefixing with the name of the service that contains the consumer.

```ruby 
class FailedPaymentConsumer
  include Hutch::Consumer
  queue_namespace "transactions"
  consume 'gc.ps.payment.failed'

  def process(message)
    mark_payment_as_failed(message[:id])
  end
end

# consumer queue name is "transactions:failed_payment_consumer" 
```