Add handling of nested configuration types via #[config(child)]

examples/simple.rs

@@ -1,6 +1,16 @@
 use std::net::IpAddr;
 use confique::{Config, Partial};
 
+#[derive(Debug, Config)]
+struct Conf {
+    #[config(child)]
+    http: Http,
+
+    #[config(child)]
+    cat: Cat,
+}
+
+
 #[derive(Debug, Config)]
 struct Http {
     #[config(default = 8080)]
@@ -8,11 +18,14 @@ struct Http {
 
     #[config(default = "127.0.0.1")]
     bind: IpAddr,
+}
 
+#[derive(Debug, Config)]
+struct Cat {
     foo: Option<String>,
 }
 
 
 fn main() {
-    println!("{:?}", Http::from_partial(<Http as Config>::Partial::default_values()));
+    println!("{:#?}", Conf::from_partial(<Conf as Config>::Partial::default_values()));
 }