I think the `cacheBust` value should be made available to a custom `fixupUrl` to maintain `fixupUrl`'s relationship to `cacheBust`. Something like:
```javascript
var fixupUrl= typeof userConfig.fixupUrl == "function"
    ? function(url){ return userConfig.fixupUrl(url, cacheBust); }
    : function(url){
        /* default implementation */
    };
```

@csnover, do you have thoughts on this?