Add missing "use strict" pragmas.

pentadactyl/content/config.js

@@ -4,6 +4,7 @@
 //
 // This work is licensed for reuse under an MIT license. Details are
 // given in the LICENSE.txt file included with this file.
+"use strict";
 
 const Config = Module("config", ConfigBase, {
     get visualbellWindow() getBrowser().mPanelContainer,