sort.go
func (s *SortFilter) By(less func(a, b string) bool) *SortFilter {
	s.add(func(a, b string) int {
		if less(a, b) {
			return -1
		}
		if less(b, a) {
			return +1
		}
		return 0
	})
	return s
}
func (s *SortFilter) By(less func(a, b string) bool) *SortFilter {
	s.add(func(a, b string) int {
		if less(a, b) {
			return -1
		}
		if less(b, a) {
			return +1
		}
		return 0
	})
	return s
}
// By adds a sort key to sort by the output of the specified less function.