White-list third-party in conf files

src/conf.rs

@@ -100,6 +100,10 @@ macro_rules! define_Conf {
                             }
                         },
                     )+
+                    "third-party" => {
+                        // for external tools such as clippy-service
+                        return Ok(());
+                    }
                     _ => {
                         return Err(ConfError::UnknownKey(name));
                     }

tests/compile-fail/conf_unknown_key.toml

@@ -1 +1,6 @@
+# that one is an error
 foobar = 42
+
+# that one is white-listed
+[third-party]
+clippy-feature = "nightly"

tests/run-pass/conf_unknown_key.rs

@@ -0,0 +1,4 @@
+#![feature(plugin)]
+#![plugin(clippy(conf_file="./tests/run-pass/conf_unknown_key.toml"))]
+
+fn main() {}

tests/run-pass/conf_unknown_key.toml

@@ -0,0 +1,3 @@
+# this is ignored by Clippy, but allowed for other tools like clippy-service
+[third-party]
+clippy-feature = "nightly"