text: Title: How to access Params from Post-Create Hook?
  Body: #### Description
  
  I am using this package with SQLAlchemy models. My goal is to use a factory to create a model instance, along with a number of associated model instances. Both specific and generic associations, like these two use cases:
  
  ```py
  # use two new gyms:
  UserFactory(gyms_count=2)
  
  # use specified gym(s)
  gym = GymFactory()
  UserFactory(gyms=[gym])
  ```
  
  I am able to use a `post_generation` hook to create the related objects, but I'm having issues accessing one of the params when doing so.  Or maybe I'm misunderstanding how the params work.
  
  #### To Reproduce
  
  ##### Model / Factory code
  
  Models (many to many association: user has many gyms, and vice versa):
  
  ```python
  class Gym(db.Model):
      __tablename__ = "gyms"
  
      id = db.Column(db.Integer, primary_key=True, index=True)
      title = db.Column(db.String, nullable=False) #> "My Gym"
  
      memberships = db.relationship("Membership", back_populates="gym")
  
  
  class User(db.Model):
      __tablename__ = "users"
  
      id = db.Column(db.Integer, primary_key=True, index=True)
      email = db.Column(db.String, index=True, nullable=False, unique=True)
  
      memberships = db.relationship("Membership", back_populates="user")
  
  class Membership(db.Model, MyModel):
      __tablename__ = "memberships"
  
      id = db.Column(db.Integer, primary_key=True, index=True)
      gym_id = db.Column(db.Integer, db.ForeignKey("gyms.id"), nullable=False, index=True)
      user_id = db.Column(db.Integer, db.ForeignKey("users.id"), nullable=False, index=True)
  
      gym = db.relationship("Gym", back_populates="memberships", uselist=False)
      user = db.relationship("User", back_populates="memberships", uselist=False)
  
  
  ```
  
  Factories:
  
  ```python
  class BaseFactory(SQLAlchemyModelFactory):
      class Meta(object):
          sqlalchemy_session = db.session
  
  class UserFactory(BaseFactory):
      class Meta:
          model = User
  
      id = Sequence(lambda n: n+1)
      email = Sequence(lambda n: f"u{n+1}@example.com")
  
      class Params:
          gyms_count = 0
  
      @post_generation
      def gyms(obj, create, extracted, **kwargs):
          if not create:
              # Simple build, do nothing.
              return
  
          if extracted:
              for gym in extracted:
                  Membership.find_or_create(gym_id=gym.id, user_id=obj.id)
  
          gyms_count = kwargs.get("gyms_count") or 0
          #gyms_count = obj.gyms_count
          # I tried `kwargs.get("gyms_count")` and `obj.gyms_count` but neither was successful.
          # how to get the gyms count param here?
  
          for _ in range(0, gyms_count):
              gym = GymFactory()
              Membership.find_or_create(gym_id=gym.id, user_id=obj.id)
  ```
  
  ##### The issue
  How to access the `gyms_count` param from within the post_generation hook? 
  
  
  
  
  
  
  
  
  
  