Allow for producing to topics dynamically.

Currently, a new producer needs to be instantiated for each topic that you want to produce to. I am working on a service that intakes messages from many producers, and then sends them on to different Kafka topics based on the content of the message. Instantiating a new producer for each topic is a lot of overhead. It would be handy if there was a way to produce to a topic by specifying topic as a parameter to a produce method.