func WithLogFunc(logf func(format string, a ...interface{})) Option {
	return func(o *Options) { o.logf = logf }
}
func WithLogFunc(logf func(format string, a ...interface{})) Option {
	return func(o *Options) { o.logf = logf }
}
// WithLogFunc sets the function that will perform message logging.
// Default is log.Printf.