Added an `options` with `:fallback` approach

This `:fallback` approach gives you the possibility to still execute your background/subscribe logic if there is an connection issue with RabbitMQ. You could use it like that:

```ruby
# Create a new instance of the message you want to enqueue.
message = Message.new(content: "Hello Mr. Watership!")

# Use Watership to enqueue it with the option `:fallback`, done.
Watership.enqueue(message: message, fallback: ->{ message.deliver })

# Your logic in your queue subscriber here.
queue = Watership.connect
queue.subscribe do |delivery_info, properties, payload|
  message = Message.new(JSON.parse(payload))
  message.deliver
end
```