filters.go
func NumberLines() Filter {
	return FilterFunc(func(arg Arg) error {
		line := 1
		for s := range arg.In {
			arg.Out <- fmt.Sprintf("%5d %s", line, s)
			line++
		}
		return nil
	})
}
func NumberLines() Filter {
	return FilterFunc(func(arg Arg) error {
		line := 1
		for s := range arg.In {
			arg.Out <- fmt.Sprintf("%5d %s", line, s)
			line++
		}
		return nil
	})
}
// NumberLines prefixes its item with its index in the input sequence
// (starting at 1) followed by a space.