Optionally disable injection of nested relations

[Patrik-Lundqvist]

It would be useful to have the ability to disable automatic injection on specific nested relations as it gives a lot of freedom how relations are handled, especially when using custom relation loading with #201.

Something like:

relations: {
  belongsTo: {
    user: {
        localField: 'owner'
        localKey: 'userId',
        inject: false
    }
  }
}

[jmdobry]

I add the inject option to relation definitions, which if set to false should probably also be paired with link: false. You can also set the inject option to a function that will be passed Resource, relationDef, instance, which you can use to manually inject the relation into the store. See https://github.com/js-data/js-data/blob/master/test/both/datastore/sync_methods/inject.test.js#L342 and https://github.com/js-data/js-data/blob/master/test/both/datastore/sync_methods/inject.test.js#L381 and https://github.com/js-data/js-data/blob/master/test/both/datastore/sync_methods/inject.test.js#L437

[Patrik-Lundqvist]

Nice, works great 👍