Do a countdown for duration, printing the template (which may accept one
	positional argument). Template should be something like
	``countdown complete in {} seconds.``

def countdown(template, duration=datetime.timedelta(seconds=5)):
	"""
	Do a countdown for duration, printing the template (which may accept one
	positional argument). Template should be something like
	``countdown complete in {} seconds.``
	"""
	now = datetime.datetime.now()
	deadline = now + duration
	remaining = deadline - datetime.datetime.now()
	while remaining:
		remaining = deadline - datetime.datetime.now()
		remaining = max(datetime.timedelta(), remaining)
		msg = template.format(remaining.total_seconds())
		print(msg, end=' ' * 10)
		sys.stdout.flush()
		time.sleep(.1)
		print('\b' * 80, end='')
		sys.stdout.flush()
	print()