Create possibility to use your own writer.

[gotpist1]

I think it would benefit a lot of people if there where a possibility to override the filename creation in TopicPartitionWriter with your own implementation. I have written my own partitioner, but i also need the files written to S3 to have another name.

My suggestion would be to add a FilenameCreator interface. In that way, one can add it's own implementation for generating a filename from the sink record. I will create a pull request for this.