Add show_round, ask_choice and play_game with injectable input and
output, and make python -m higher_lower play a game. Each account is
shown once per game; a wrong guess or running out of accounts ends it.
Invalid input is re-asked. Ctrl+C and Ctrl+D leave with a goodbye.
Cover the loop with scripted tests (36 passing).
Task: MIL-003#1
Task: MIL-003#2
Task: MIL-003#3
Task: MIL-003#4
Task: MIL-003#5
Closes#12Closes#13Closes#14Closes#15Closes#16