It looks like output format and output stream are very strongly tied together. Specifically, the command line interface has global configuration options for output-type (i.e., stdout, kafka) and format(i.e, json, avro). Sending JSON to Kafka should be a valid output configuration, but this is not supported in the current code. Output types should be generic regardless of the format. For example, in addition to stdout and Kafka, I could envision a file as another output type.

Alrighty, I've made those changes. JSON/avro are interchangable for all output types, and output types now include file.
Maybe in the future we can abstract the output streams into io.Writer so that we don't have to explicitly handle every combination of types/destinations.

I closed this for now as I need to test it a little more.