Enhancements

• Consumer: Add support for printing message headers (-f ".. %h.. \n")
• Producer: Add support for specifying message headers (-H name=value)
• Emit current controllerId in metadata output, if supported
• Add -F <file> to read config from file. Defaults to $KAFKACAT_CONFIG or ~/.config/kafkacat.conf (#130). With some support for translating Java config properties to librdkafka counterparts.
• Support Fixed key (-k ..) (#123, @solsson)

Fixes

• Make sure topic configuration is applied first (#149)
• JSON: emit NULL keys and values as null rather than empty string
• Fix tstype json output (@hustclf, #166)
• Honour -c count in consumer mode by yielding