logger.go
func (l *Logger) Fatalf(format string, args ...interface{}) {
	l.log(CRITICAL, &format, args...)
	os.Exit(1)
}
func (l *Logger) Fatalf(format string, args ...interface{}) {
	l.log(CRITICAL, &format, args...)
	os.Exit(1)
}
// Fatalf is equivalent to l.Critical followed by a call to os.Exit(1).