Not able to use standalone & require flags at the same time

As notified at https://github.com/substack/node-browserify/issues/939, I'm not able to generate a bundle that at the same time can be used standalone and also export its content so it can be required by other bundles, that it's useful for frameworks that can use build-in and external plugins at the same time. Reviewing the code, [there's a conditional that explicit forbit this use case](https://github.com/substack/browser-pack/blob/master/index.js#L44-L52), so I would like to know if it's on purposse and why, or if this could be fixed for this use case (I think it's possible, but needs to investigate a little bit more the code...).