Interact with Environment Groups via CLI #32

This is moving this past request from the deprecated render-cli project to here:
render-oss/render-cli-deprecated#54

We often used the heroku config:get command to retrieve environment variables stored remotely.

It would be great if there was an equivalent way in the render-cli to retrieve environment variables from particular Env Groups.

(or if this already exists somewhere...I'd love to know where it is 🙂)