Figure out something for non-flat configs

[mythmon]

It could be nice to have a nested configuration, like

type Config = {
  aws: {
    accessKeyId: string;
    secretAccessKey: string;
    defaultRegion: string;
  }
}

Ganzu should provide some way to specify those keys, in both rich formats like YAML and string based formats like environment variables.

Note that this is different than map values (#5). A map value is an unknown set of keys and values, whereas this is more like a struct: a known set of named children.