implement 'resume' feature for a running game

this commit implement a feature to pause a running game session and resume it again.
the game will start from where you left it.

on pressing `spacebar`, a running game session will get paused and game's score and position data will be saved.
command `term2048 --resume` will resume the game and start it from where you paused the game.
We should signal the error someway, for example by returning `True` if the game was successfully saved, and `False` if not. The caller would then be able to write an error message on failure.