Add option for test_app to test models with relations to different package models.

[pydanny]

1.Either add option in cookiecutter.json or do it by default and provide instructions for deletion if not needed. 2. Provide something like https://github.com/pydanny/dj-spam/commit/a2700eb528c0f76953e5d7e4e92a8c84f2e2caf7 but more generic. 3. In the generated README, include instructions for using and extending it (for example, how to generate migrations).