title: Chrome extension to modify page's script includes and JS
body: <p>I work on a javascript library that customers include on their site to embed a UI widget.  I want a way to test dev versions of the library live on the customer's site without requiring them to make any changes to their code.  This would make it easy to debug issues and test new versions.</p>
  
  <p>To do this I need to change the script include to point to my dev server, and then override the load() method that's called in the page to add an extra parameter to tell it what server to point to when making remote calls.</p>
  
  <p>It looks like I can add JS to the page using a chrome extension, but I don't see any way to modify the page before it's loaded.  Is there something I'm missing, or are chrome extensions not allowed to do this kind of thing?</p>