#615 - allow specifying a listener for partition assignment changes

[nachogiljaldo]

Closes https://github.com/segmentio/kafka-go/issues/615

Adds a listener to send re-assignments to a listener. After this, it is possible to possible to provide a listener (optional) to have consumers be notified of changes in partition assignments for the current consumer.

This is useful for debugging as well as potentially other things such as dropping in-flight events for partitions that are not owned anymore.