Add external_edit_file_suffix to config (#253)

src/tests.rs

@@ -186,6 +186,7 @@ pub fn mock_tunables() -> TunableValues {
         .into_iter()
         .collect::<HashMap<_, _>>(),
         open_command: None,
+        external_edit_file_suffix: String::from(".md"),
         username_display: UserDisplayStyle::Username,
         message_user_color: false,
         notifications: Notifications {