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.
Added in recent commit. check this out now, please :)