Basic stopwatch

<Enter> to start/pause/resume.

reset to reset stopwatch and start from 0.

<Ctrl-d>(EOF) or exit to exit.

Any other string input at the prompt has no effect and does not affect the stopwatch; we can use this to add short notes on what we're recording.

To-dos

Save data to file.
Animated display of elapsed time instead of only showing the time when the stopwatch is stopped.
Alarms that sound after a preset length of time.
Multiple stopwatches running in parallel.