small patch for using a custom .js file

The google_cse.js file applies a google watermark feature to the search form which only gets loaded when the javascript gets loaded. Therefore I've made a small patch that adds an admin setting for specifying a custom google_cse javascript file. It's only for the 5.x version of google_cse, but can easily be made for 6.x I'd say. There might be a reason why such a feature doesn't exist already - if so feel free to explain, thanks!