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'm going to make a first pass at some [coding standards](https://github.com/audreyr/cookiecutter/issues/219), but I'd prefer if we didn't add any more ambiguity, [quote wise](https://github.com/audreyr/cookiecutter/issues/204). Are you cool with keeping things to single quotes and updating this module while you're here??