multi.go
func MultiLogger(backends ...Backend) LeveledBackend {
	var leveledBackends []LeveledBackend
	for _, backend := range backends {
		leveledBackends = append(leveledBackends, AddModuleLevel(backend))
	}
	return &multiLogger{leveledBackends}
}
func MultiLogger(backends ...Backend) LeveledBackend {
	var leveledBackends []LeveledBackend
	for _, backend := range backends {
		leveledBackends = append(leveledBackends, AddModuleLevel(backend))
	}
	return &multiLogger{leveledBackends}
}
// MultiLogger creates a logger which contain multiple loggers.