Build Web Components with a shared bundle

It would be incredibly useful if I the widget build could support building multiple web components in a project and output a _shared bundle_ to limit the total size of the components.

Right now if I have multiple elements in my `.dojorc` like this.

```
{
"build-widget": {
"elements": [
"src/widgets/DatePicker",
"src/widgets/Authentication",
"src/widgets/Profile"
]
}
}
```

Each output web component contains it's own widget source, but also all the `dojo` code needed as well in its bundle.

It would be really useful if we could build the whole project of web components to use a shared `common.js` file of the `dojo` code used. I assume it could work the way webpack dynamically loads files at runtime to load the common file as needed per web component. This helps cut down on the total size of js transferred.

The real benefit here is that at this point `dojo` could be used a source for building scalable web component libraries that could be used across multiple projects, regardless of the technology or framework being used by developers.