Added a MenuGameState, documented it, and started to use it in the game loop. It is far from done.

 - Added fully documented MenuGameState.h and MenuGameState.cpp files.
   - These files add a MenuGameState class inheriting from the ABC class: GameState.
 - Told cmake about MenuGameState.cpp so it will compile with ultimate_pong.
 - Set the initial game state of the game loop to our new MenuGameState.
 - Set up MenuGameState's update function to exit on SDL_QUIT and SDL_KEYPRESS.