Display / set / update the dtool cache directory.

def cache(cache_directory_path):
    """Display / set / update the dtool cache directory."""
    if not cache_directory_path:
        click.secho(dtool_config.utils.get_cache(
            CONFIG_PATH,
        ))
    else:
        click.secho(dtool_config.utils.set_cache(
            CONFIG_PATH,
            cache_directory_path
        ))