Add support for JSON-format output from all plugins

It might be nice to support JSON output as a valid format for all custom plugins, so that they can be piped through HTTP/JSON rather than relying solely on NRPE or similar.

This would probably be supported via something like the following pattern in the main() function for each plugin:

def main(...):
    ...

    status, message = check_thing(...)

    if args.json:
        utils.write_json(
            status,
            message,
            validity=3600,
        )
    else:
        print(message)
    return status