Generate model instance with related data at once.

Thanks for the great library.

Is there an api to generate a model instance with several relations (OneToOne, ManyToOne, ManyToMany)? Like so:
```python
user = mixer.blend(User)
.with(Profile, 1) # one profile
.with(Post, 4) # 4 posts
.with(Comment, 20, post=mixer.mix.posts) # 20 comments to previous posts
```

Start by reviewing Mixer’s existing fixture-generation APIs and how they support Django, Flask, SQLAlchemy, and custom Python objects. Define the supported relation combinations and the chaining behavior requested in the example; done means related instances can be generated together with predictable counts and links across those integrations.

Written by the indexing model from the issue text.