~/.bin/working-time.go: Support logging breaks

You can even specify "more" time by using a negative break time...

What's still not supported is registering suspends as breaks.  (Which I
need at home, but not at work...)

.bin/working-time.go

// Day contains information about how long you've worked today.
type Day struct {
	Start time.Time `json:"start"`
	Start time.Time     `json:"start"`
	Break time.Duration `json:"break,omitempty"`
}
// modes:
		writeDay(dayFile, day)
	}
	dur := time.Since(day.Start)
	if len(os.Args) > 2 && os.Args[1] == "break" {
		dur, err := time.ParseDuration(os.Args[2])
		if err != nil {
			fmt.Fprintln(os.Stderr, err)
			os.Exit(1)
		}
		fmt.Fprintf(os.Stderr, "took a %s break\n", dur)
		day.Break += dur
		writeDay(dayFile, day)
	}
	dur := time.Since(day.Start) - day.Break
	hours := dur.Hours()
	minutes := dur.Minutes() - float64(int(hours)*60)
	fmt.Printf("%d:%02d\n", int(hours), int(minutes))