Move variables and triggers to be defined in the database

* Variables are now defined in the database, rather than the game
  itself.
* Triggers are now defined in the database, rather than hidden away in a
  method in the game. Custom triggers can now be added as well, in case
  a game needs more complex behavior that isn't necessarily available in
  the base library.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

examplegame/vars.py

@@ -1,8 +1,7 @@
 """
 Define all game variables here.
 """
+from . import database
 
-vars = {
-    "cabin_door_open": False,
-    "mailbox_open": False,
-}
+database.add_var("cabin_door_open", False)
+database.add_var("mailbox_open", False)