[DX] Re-work generate:bundle for simple app bundles #290

@weaverryan please review my pull request ;)
I think we could expand a little bit this explanation (needs reword from a English native):

```
* ``--shared``: set this option if you are creating a bundle that will be shared
  across several of your applications or if you are developing a third-party
  bundle. Don't set this option if you are developing a bundle that will be
  used solely in your application (e.g. ``AppBundle``).
```