Improve Tank-class and add support for 2nd player
Improve Tank-class and add support for 2nd player

-Add separate controls for p1 and p2
-Add tank for player 2 for testing
-Limit the amount of bullets each tank can fire at a time
-Implement causing damage to tanks
-New GameObjects are always added to the global game_objects list in constructor
-Tank's color varies on its current hp.
-End round when one of the tanks is destroyed
-Rename global variables in uppercase
-Spawn a ring of bullets when a tank is destroyed
-Add a check if tank exists before attempting to add ammo to it
-Make bullets spawn closer to the tank

debnera/js_tank_game,debnera/js_tank_game