Support json output for CLI commands

[deliahu]

Description

Add a flag (e.g. --json) to certain CLI commands to output the response in structured json format.

Questions

• Which commands should support this? Definitely get; any others?

suggested by #1160

[RobertLucian]

A possible alternative to mentioning the output format is: have an -o/--output option, with the allowed values being json, yaml, and text. text would be the default value so that the current output format is being kept.

[zouyee]

/assign

[deliahu]

Thanks @zouyee, yes this one is up for grabs; let us know if you have any questions about this or if you'd be interested in discussing any of the design.