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

[IKenTN]

Some information:

• I used a map because I needed to save the food spawns to enable global visibility (I assume every map has a different food spawn mechanism, but I only needed to tweak the standard one slightly)
• I have now set the view range to 5 as standard (it was previously adjustable with a flag), since I also edited the visualisation inside the board repo and needed information on the view range (If you have any suggestions on how to incorporate this information into line 205 of play.go without making significant changes to the rest of the file, please let me know)