add a alien target and a shooter to hit it

I am actually really proud that i was able to figure out how to make the alien move
in every direction randomly.

proposal.md/nav_game.py

@@ -1,9 +1,14 @@
 from retro.game import Game
 from spaceship import Spaceship
-from targets import Asteroid
+# from asteroid_spawner import AsteroidSpawner
+from targets import Alien
+from shooter import Shooter
+import random
 
 board_size = (100, 25)
+width, height = board_size
 ship = Spaceship(board_size)
-targets = Asteroid((board_size[0] // 2, 0))
-game = Game([ship, targets], {"score": 0}, board_size=board_size, color="white_on_blue")
+shooter = Shooter() 
+spawner = Alien((random.randint(0, width-1), random.randint(0, height-1)))
+game = Game([ship, spawner, shooter], {"score": 0}, board_size=board_size, color="white_on_blue")
 game.play()