Add exclude model option to dump_object

[paulogiacomelli]

Adding a --exclude-models similar to exclude-fields. This allows on our use case to exclude some models that we know it should not be included in the dump_object fixture.

There wasn't a contribute guide, but I tried following the code base flow and included a test.

Example Usage:

./manage.py dump_object app_name.model_name 1 --kitchensink --exclude-models app_name.model_name1 app_name.model2