Get the config file location from $COOKIECUTTER_CONFIG, if set

Apart from being useful for users, this will allow us to test more easily, without having to save/restore the user's config as much as we currently do.
BTW, I prefer double quotes for ```"""..."""``` blocks (as it makes them stand out more), so I'm leaving them. The only files using ```'''``` are ```hooks.py``` and ```test_hooks.py```.