[Idea] Add option to display command outputs as JSON

[brouberol]

I'm writing a script automating the restart of a kafka cluster, using kafkat restart-cluster at its center. It's calling kafkat commands via subprocesses, and needs to parse the output. It would be super convenient to have a --json (or --format=json) flag we could pass to the command-line to make it output JSON instead of human-readable text.

What do you think?

Thanks!