Feature request: Make reporting queue ID possible

Hello,

Here a sysadmin who sees often mail server log files.

It would be good thing if mailin would return more then just 200 OK at data_end. That "250 with text" is possible.

As patch (no idea if the approach is correct)

--- a/mailin/src/lib.rs
+++ b/mailin/src/lib.rs
@@ -120,6 +120,12 @@ pub trait Handler {
         response::OK
     }
 
+    /// At end of receiving data returning a 250 string,
+    /// typical use case is reporting a queue id
+    fn data_end_250_text(&mut self, _text: &str) -> Response {
+        response::OK_250(_text)
+    }
+
     /// Called when a plain authentication request is received
     fn auth_plain(
         &mut self,