Users should be able to update the topic on the writer to a new topic

[DanWertheimer]

For example, if I want to write raw data and processed data onto Kafka, into two separate topics, but with the same producer config and schema registry config.

Suggested API

from py2k.writer import KafkaWriter

writer = KafkaWriter(topic='topic1',
                                   producer_config=some_producer_config,
                                   schema_registry_config=some_sr_config)
writer.write(some_records)

writer.topic = 'new_topic'
# OR
writer.update_topic('new_topic')

writer.write(other_records)