Add #[config(partial_attr(...))] to add attributes for partial type

CC #17

examples/simple.rs

@@ -15,6 +15,7 @@ struct Conf {
 
 /// Configuring the HTTP server of our app.
 #[derive(Debug, Config)]
+#[config(partial_attr(derive(Clone)))]
 struct Http {
     /// The port the server will listen on.
     #[config(env = "PORT")]