Specify required environment variables and secrets within config

Sometimes I deploy without remembering to set some env vars so it would be useful for wadsworth to skip deployment completely if there are missing env vars.

```js
E({
        name: "some-app",
        url: "...",
        vars: ["SUPER_SECRET_REQUIRED_PASSWORD"],
        up: ["docker-compose", "up", "-d"],
        down: ["docker-compose", "down"]
})
```