Add .es to register extensions

.es is a known ecmascript file extension and should be handled by the loader
See RFC 4329, 8.2.

lib/6to5/register.js

@@ -123,7 +123,7 @@ var hookExtensions = function (_exts) {
   });
 };
 
-hookExtensions([".es6", ".js"]);
+hookExtensions([".es6", ".es", ".js"]);
 
 module.exports = function (opts) {
   // normalise options