Allow NestedModel and Object editors' form options to be extended/overridden.

I wanted to be able to override the fields rendered and template used by the nested form.

Contrived example:
```javascript
new Backbone.Form({
  schema: {
    address: {
      type: "NestedModel",
      model: Address,
      formOptions: {
        template: JST["address_form"],
        fields: ["street_address", "locality", "postal_code", "region", "country"]
      }
    }
  }
});
```