💡 Feature description

In config-rs, there's an option to define a separator which allow users to structure their env vars. I think an example will illustrate better:

#[derive(Deserialize)]
struct Config {
    database: Database
}

#[derive(Deserialize)]
struct Database {
    name: String,
}

If we define a separator, let's say "__", we could create this Config by setting DATABASE__NAME=foo.

Would it be possible and desirable for envy to include it?

Alternatives

It is possible to get the same behavior using the flatten attribute from serde (as noticed in #15). The drawback is that it demands a good amount of boilerplate because we would have to serde's rename every field in the nested struct if we wanted to use this separator idea.