Allow controller to override its `req.model`.

Allows controllers to override the model id passed into `req.model`:

```javascript
// api/controllers/ThisController.js
module.exports = {
  _config: {
    model: 'that'
  }
}
```
Would enable blueprint routes for `/this/*`, using a hypothetical model `That`.

Most prominent use case would be a short-term solution for namespaced controllers missing out on all blueprint functionality as there is no possibility to namespace models at the moment.