EnableExpandEnv make it can access environment variable in config content.

Load config # load by default (local file driver): go run main.go # load from local file driver: go run main.go -config-driver local -config config.yaml # load from pre-registered remote driver: go run main.go -config-driver redis -config config.yaml # load from dynamic created remote driver: go run main.go -config-driver consul -config-script consul.yaml -config app/config.yaml.