Keep up with .sample.env

[jyurek]

We have an app that uses env vars, and when we add a new one, we add it to .sample.env. We obviously can't add it to .env. It'd be useful to have envy track differences there and if a new key is in .sample.env then add it to .env (or complain about it so we see it and can handle it).

[BlakeWilliams]

I love this idea.

I'd happily accept a PR if anyone wants to start on it, if not I can look into working on it soon.

[jyurek]

I was mostly leaving this for myself so I don't forget, but I'm not opposed to anyone else doing it.