SubString for go (works well with unicode)

// SubString works well with unicode
func SubString(str string, start int, length int) string {
    r := []rune(str)
    n := len(r)
    if start >= n || length <= 0 {
        return ""
    }

    if start < 0 {
        start = 0
    }
    end := start + length
    if end > n {
        end = n
    }

    return string(r[start:end])
}