durafmt.go
func ParseString(input string) (*Durafmt, error) {
	if input == "0" || input == "-0" {
		return nil, errors.New("durafmt: missing unit in duration " + input)
	}
	duration, err := time.ParseDuration(input)
	if err != nil {
		return nil, err
	}
	return &Durafmt{duration, input, false}, nil
}
func ParseString(input string) (*Durafmt, error) {
	if input == "0" || input == "-0" {
		return nil, errors.New("durafmt: missing unit in duration " + input)
	}
	duration, err := time.ParseDuration(input)
	if err != nil {
		return nil, err
	}
	return &Durafmt{duration, input, false}, nil
}
// ParseString creates a new *Durafmt struct from a string.
// returns an error if input is invalid.