Allow adding newly created Elements directly as Relationship to an entity

I love to have a possibility to add a newly created entity straight away to another Entity as Relationship.

I my mind is something like:
```
setItem(data, { attach: { to: 'myOtherStore', id: 'myId', as: 'relationshipName' } })
```
which then fires a update mutation in the targeted store.
This should also be possible with an Array to add it to several elements