Allows _quantity use on recipes containing OneToOne fields (#145)

[Honza-m]

Fixes #145 by adding an one_to_one argument to foreign_key and documents the behaviour.

When one_to_one=True, the make_recipe function will create an unique related model for each _quantity. This is False by default so the behaviour where only one related model is created remains the default.

Thanks for all the 🍰 😄

[berinhard]

This is great @Honza-m! I really liked the strategy you've implemented on adding a new boolean parameter to the foreign_key method. Nice touch with the docs too. Thanks for your contribution!

[tuky]

Thank you all 👍