Update design_prototype_decent.py

Added game mode.. But there appears to be a bug !!!

Test // | Game | Help | Quit
Welcome to the application
>>> Game
Test // | Main | Help
Let us play this game
>>> help
Test // | Game
This is how you use this application
>>> game
Test // | Help | Help
Let us play this game
>>> main
Test // | Help | Help
Let us play this game
>>> Bye bye