Added kwargs to ModelOptions for further customization
I added another commit that removed the forced auto_increment no matter what. I feel like people should be able to set the pk on an item even if there is an auto_increment.
 Both of your changes should now be merged in (more or less).  Thank you!

This allows people to add more options to the model, for example if used with a serializer that looks for excluded fields:

```
class User(Model):
    class Meta:
        excludes = ['id']
```

The kwargs that would be used is unique for each use case.
