Support nested models

You currently have to jump through a few extra hoops if you want to have torch forms for e.g. a model on the other side of a `has_many` relationship. I'm not sure how practical this would be, but I wonder if it would be possible to do something like:

```
mix torch.gen eex Admin Post:Comment comments body:text inserted_at:date
```

and have it do the right thing(s).
