Added optional secondary formatter: format.key

Config property: format.key = <format string>
Config property: format.key.type = string|json

If output is kafka the key will be used as message key.
If output is string it will simply output an additional line rendered
according to the format.key format and format.key.type.

Change-Id: Icd97a6d3f1c67e12dbfa849796c0ac8807a50b1f