cf-key --show-hosts in JSON

It would be nice if cf-key --show-keys could output in JSON format.

Recently the --no-truncate option was proposed to cf-key as a way to prevent truncation of hostnames output by cf-key --show-keys. That change makes cf-key -s output more friendly for interacting with shell tools like grep.

Adding support for JSON output would be nice for aligning with cf-promises --syntax-description json, cf-promises --policy-output-format json, cf-agnet --show-evaluated-vars and make integrating with higher level tools easier.

Note: It's possible to achieve this by writing policy to leverage `hostsseen()` and render a JSON file.