Support for kt: tags in plog-kafka

[pcarrier]

• If a message has kt:foo and kt:bar tags, forward the message to the foo and bar topics.
• Configuration now specifies a default topic, used for messages that do not have such tags.
• If the default topic is not set, a warning is printed on startup and messages are ignored (allows for more clever pipelines)
• The magical value null can be used for the default or tag to avoid sending to Kafka, for example for warmup or to use the same port for multiple sinks and client-side routing.

[nelgau]

Yup! 👍