Build: Add a comment explaining why the es3 option is needed

It might not be obvious to everyone that IE 9 & Android 4.0 are not
ES5-compliant browsers (by a large margin) so it's better to add a support
comment. This requires slight changes in parsing the config file
as it's not a pure JSON anymore. JSHint understands such files without
problems.

(cherry-picked from 669cb16d76)

Closes gh-2520

src/.jshintrc

@@ -12,6 +12,9 @@
 
 	"sub": true,
 
+	// Support: IE < 10, Android < 4.1
+	// The above browsers are failing a lot of tests in the ES5
+	// test suite at http://test262.ecmascript.org.
 	"es3": true,
 	"browser": true,
 	"wsh": true,