Add ability to generate nested belongs_to associations.

[keshavbiswa]

If you're seedie.yml looked like this:

parent_model:
  attributes:
    some_attribute: 'hello'
child_model:
  associations:
    belongs_to:
      parent_model: 'random'

This would work. But if it's the other way round:

parent_model:
  attributes:
    some_attribute: 'hello'
  associations:
    has_many:
      child_model: 2

This would only work if Child model is not a Join Table or if there isn't another required belongs_to association.

This PR fixes this by adding ability to write nested associations for belongs to

Now you can write:

parent_model:
  attributes:
    some_attribute: 'hello'
  associations:
    has_many:
      child_model:
        attributes:
          title: 'some title'
        associations:
          belongs_to:
            another_parent_model: 'random'