countdown_loop: Runs a Pomodoro timer.
>>>
result <- countdown_loop no_delay "00:00"
...
>>>
result
"00:00"
>>>
result' <- countdown_loop no_delay "00:59"
...
>>>
result'
"00:00"
>>>
result'' <- countdown_loop no_delay "01:59"
...
>>>
result''
"00:00"