output: require message encoder

Added requirement for an encoder.

- https://github.com/mozilla-services/heka/blob/671a9df6c99bd3f778f5e84cc3a6eb3af71de856/plugins/http/http_output.go#L90-92
- https://github.com/mozilla-services/heka/blob/42eddff46b7ae73b427ef552ccec614dd5e6c38c/plugins/kafka/kafka_output.go#L309-311

The encoder may mutate the pack, but we don't use the encoder's outBytes directly since we need to get fields like Payload and Severity to send to hipchat.