Update example game some, update engine some

* Add RevealAction/UnrevealAction for revealing/hiding items in a room
* Add a lot of checks for items being revealed when it's attempted to be
  triggered
* Implement TeleportAction (mostly)
* For all Check* family of actions, the `yes` and `no` values may be
  just be a single action instead of an array of actions
* Change up how room descriptions and stuff work, mostly so that you can
  specify multiple lines in an array so you can preserve paragraph
  breaks when displayed.
* Example game has some more content

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

examplegame/vars.py

@@ -3,5 +3,6 @@ Define all game variables here.
 """
 
 vars = {
-    "cell_door_open": False,
+    "cabin_door_open": False,
+    "mailbox_open": False,
 }