Add supper for Extension and  Framework targets

- Adding a framework target to the main project: builds, embeds and (optionally) loads the framework to the main app.
- Adding a framework target to a extension project: only builds and loads the framework.

You could want a framework to be shared by 2 extensions but *not* loaded by the main app. You would declare de target in the 3 projects but select `load: false` in the main project.

Is this behavior OK? Should probably be documented in the blog post or dev center documentation.