Filter to enable imperfect information for standard mode

[IKenTN]

I've implemented an imperfect information mode for the standard map (only currently). I'm fairly new to GO, but I've tested it from the snake side too and it seems to work as intended.
New features:

• a new -i flag to view the radius for all snakes
• saves food in gamestate and shows it globally on the spawning turn for all snakes
• filters all information for each snake and only returns things within the view range