addSourceURLs

Hello TJ, little proposition for **addSourceURLs**
most of my packages use index.js as filename thus when i use  addSourceURLs
and check the sources tab in chrome aIl I see is just see a bunch of index.js files
What do you think about adding this optional mapping function  so I can use something like this to search file more easily 
```js
  builder.addSourceURLs(function(file) {
    return file.replace('/', '-');
  });
```