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.
I wondered about that. "path to" seemed to me to imply that it was the name of the directory containing the file, rather than a full pathname, but equally on rereading "name of" suggests you don't specify a directory. Or maybe be explicit and use "pathname of"?