Use `conf` package for CLI config file

[garrettjstevens]

I'd like for us to investigate the possibility of using the conf package to manage our CLI's config file. It's a widely-used package, and it provides some potentially useful features we may want in the future, such as config file version migrations (in case we need to change something about the config file in the future after a public release).

[garrettjstevens]

@dariober I looked at a couple of the things we talked about at our pairing today regarding conf.

• I couldn't reproduce the error with the custom cwd and fileExtension. Did you perhaps somehow accidentally create a directory called config.yaml?

• It looks like conf doesn't allow us to add top-level properties to the schema (see https://github.com/sindresorhus/conf/issues/80), but since the schema is optional, I think we can continue with using conf without a schema and add in our own validation (we can probably keep the existing joi validation). If we subclass Conf, we can add our own validation like this:

  class ApolloConf extends Conf {
  constructor(...options: ConstructorParameters<typeof Conf>) {
  super(...options)
  // validate this.store
  }
  
  get store() {
  const superStore = super.store
  // validate superStore
  return superStore
  }
  
  set store(newStore) {
  // validate newStore
  super.store = newStore
  }
  }

[dariober]

Did you perhaps somehow accidentally create a directory called config.yaml

Yes! It must have happened while playing with Conf

---

In branch use-conf I replaced the old implementation with the Conf package.

[garrettjstevens]

Done in 3cfca9f60837aa9d601cb10efb6a246890d9c5b8