Allowing for projects to define where they put their load_{model/dataset} function defintions.

[Lucaweihs]

Currently we require that loaded projects place their load_dataset/load_model definitions in a main.py script in the top-level directory. This can be a bit limiting if we want to load data from an established project that uses a different directory structure (e.g. the robothor objectnav challenge). This update allows such a project to define a .prior-config.json file that can be used to specify a relative path to a python file which is used instead of ./main.py.