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

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
```python
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)
```