Fixes #6 + bonus: testing_proj game_flags no longer has fatal errors
- Removes call to pygame.mixer.init() from engine.py and moves it to
sound.py
- Adds SoundController ABC and renamed SoundController to
DefaultSoundController
- Added NullSoundController implementation of SoundController
- Added function to try initializing mixer via pygame.mixer.init()
and initializes _soundthread with NullSoundController if it fails