Add --pod support for adapters, serializers, and transforms

Adds `--pod` support for adapters, serializers, and transforms, so they may be grouped together with their respective models. This also allows grouping for models and their supporting files (adapters, serializers, and transforms) that don't necessarily have an associated route. 

For example, a model `taco` with the `--pod` flag set:

```
ember g model taco --pod
ember g adapter taco --pod
ember g serializer taco --pod
ember g transform taco --pod

// produces the following, plus associated tests
app
└ pods
   └ taco
      ├ model.js
      ├ adapter.js
      ├ serializer.js
      └ transform.js
```