Support to define THUMBNAIL_OPTIONS_DICT on per model basis

Some time we need to define different sizes based on different model. I have added the patch which will give higher importance to model level `THUMBNAIL_OPTIONS_DICT` if that is not defined always fall back to the `settings.OPTIONS_DICT`. How to use it:
```
class Image(AsyncThumbnailMixin, models.Model):
    image_field_name = 'image'
    THUMBNAIL_OPTIONS_DICT = {
        'small': {
                'geometry': '30x30',
                'crop': 'center'
        },
    } 
```