[FR] json output for `proxy list`

[proItheus]

Thanks for this awesome project! I want to use clashctl in some of my scripts, but parsing the output of proxy list is a pain.

Maybe we can add a json output option for easy parsing. e.g. with a schema like:

{
  "name": "proxygroup1",
  "type": "Selector",
  "delay": null,
  "subproxies": [
    "proxy1",
    "proxy2",
    "proxygroup2"
  ]
}

[George-Miao]

Sorry for such a late response. Currently I'm not using clash nor clashctl anymore, which defeats the single most important reason to continue adding features for this project. I'm still willing to accept PR's and new contributors though, so if you think this is necessary, please kindly write a PR and I will be more than happy to review and merge it.