slices.go
func (s StringSlice) IndexOfIgnoreCase(str string) int {
	str = strings.ToLower(str)
	for i, v := range s {
		if str == strings.ToLower(v) {
			return i
		}
	}

	return -1
}
func (s StringSlice) IndexOfIgnoreCase(str string) int {
	str = strings.ToLower(str)
	for i, v := range s {
		if str == strings.ToLower(v) {
			return i
		}
	}

	return -1
}
// IndexOfIgnoreCase looks for specified string, disregarding letter casing,
// into current slice.