~/.bin/working-time.go: Support tracking duration of breaks

You call `working-time break` at the start and end of the break, and it
calculates how long your break was.

.bin/working-time.go

// Day contains information about how long you've worked today.
type Day struct {
	Start time.Time     `json:"start"`
	Break time.Duration `json:"break,omitempty"`
	Start      time.Time     `json:"start"`
	Break      time.Duration `json:"break,omitempty"`
	BreakStart *time.Time    `json:"break-start,omitempty"`
}
// modes:
		writeDay(dayFile, day)
	}
	if len(os.Args) > 2 && os.Args[1] == "break" {
	if len(os.Args) == 2 && os.Args[1] == "break" {
		if day.BreakStart != nil {
			dur := now.Sub(*day.BreakStart)
			day.Break += dur
			day.BreakStart = nil
			fmt.Fprintf(os.Stderr, "took a %s break\n", dur)
		} else {
			day.BreakStart = &now
		}
		writeDay(dayFile, day)
	}
	if len(os.Args) == 3 && os.Args[1] == "break" {
		dur, err := time.ParseDuration(os.Args[2])
		if err != nil {
			fmt.Fprintln(os.Stderr, err)