SubString for go (works well with unicode)

 1// SubString works well with unicode
 2func SubString(str string, start int, length int) string {
 3	r := []rune(str)
 4	n := len(r)
 5	if start >= n || length <= 0 {
 6		return ""
 7	}
 8
 9	if start < 0 {
10		start = 0
11	}
12	end := start + length
13	if end > n {
14		end = n
15	}
16
17	return string(r[start:end])
18}