Automatically detect name for nested structs

[dorianim]

I love this lib, simple and straight forward, thanks!

Just one thing that would be nice to have:
In a case like this:

#[derive(Envconfig, Debug)]
struct ProxyAuthConfig {
    enabled: bool,
    headers_username: Option<String>,
    headers_password: Option<String>,
}

#[derive(Envconfig, Debug)]
struct LocalAuthConfig {
    enabled: bool,
}

#[derive(Envconfig, Debug)]
struct AuthConfig {
    #[envconfig(nested = true)]
    proxy: ProxyAuthConfig,
    #[envconfig(nested = true)]
    local: LocalAuthConfig,
}

#[derive(Envconfig, Debug)]
pub struct ConfigurationService {
    jwt_secret: String,

    #[envconfig(nested = true)]
    auth: AuthConfig,
}

the auth.local.enabled would be named AUTH_LOCAL_ENABLED by default :)