Make the unrooted_must_root conditional on a default Cargo feature.

Only http://perf.rust-lang.org/ will disable it,
in order to be less subject to changes to rustc internal APIs.

components/script/lib.rs

@@ -16,6 +16,7 @@
 #![doc = "The script crate contains all matters DOM."]
 
 #![plugin(script_plugins)]
+#![cfg_attr(not(feature = "unrooted_must_root_lint"), allow(unknown_lints))]
 
 extern crate angle;
 extern crate app_units;