Callback that is used whenever --config is passed.

def read_config(ctx, param, config_path):
    """Callback that is used whenever --config is passed."""
    if sys.argv[1] == 'init':
        return
    cfg = ctx.ensure_object(Config)
    if config_path is None:
        config_path = path.join(sys.path[0], 'v2ex_config.json')
    if not path.exists(config_path):
        sys.exit("Can't find config file at {0}.\nPlease read "
                 "https://github.com/lord63/v2ex_daily_mission "
                 "to follow the guide.".format(config_path))
    cfg.load_config(config_path)
    return config_path