support es module output via browser-pack

This is relevant for the "browser-pack" module, and specifically the prelude wrapper.
The issue/decision to make is probably just where the best place to implement this in the browserify settings and then match that in browser-pack? With a very small additional option to configure to browser-pack's prelude, browserify can easily easily support this:
```html

```

all we'd have to do is conditionally wrap the original browser-pack prelude with an export default iife. it works no problem, I've tested locally. The benefit of this might not seem apparent, but its actually a huge win for multi frame apps. When you import stuff as a module(in chrome at least), it will share the instance of the import across frames, similar to built in objects like Array. This can reduce the memory impact of multiple iframes using the same libraries from MBs to KBs.