[FEAT] gk pr list --json

[hans-d]

Is your feature request related to a problem? Please describe.
would like to get the output in a machine readable format for further processing, eg to list specific fields only. Eg to use in WTF

Describe the solution you'd like
all output can have a --json or so like flag

Describe alternatives you've considered
scraping with sed/awk

Additional context
havng some additional info in the json (optionally), like tags, author, would be nice extra