Adding a function to piwik.js to hook an external js file that can be fired when piwik.js is not

Adding a new function to piwik.js in order to be able to hook an external js file that could be fired when piwik.js is not fired. 
E.g. to track scrolls, piwik.js is not fired. In order not to rewrite the entire getRequest method from piwik.js, add the possibility to hook an external js file which takes care of this new tracking and can then use the core to send the request.