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/asteroid.py

@@ -0,0 +1,19 @@
+class Asteroid:
+    character = 'O'
+
+    def __init__(self, position):
+        self.position = position
+
+    def play_turn(self, game):
+        width, height = game.board_size
+        if game.turn_number % 2 == 0: 
+            x, y = self.position
+            if y == height - 1: 
+                game.remove_agent(self)
+            else:
+                ship = game.get_agent_by_name('ship')
+                new_position = (x, y + 1)
+                if new_position == ship.position:
+                    game.end()
+                else:
+                    self.position = new_position