Add "help" trigger and help() static method to trigger objects

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

agame/game.py

@@ -33,6 +33,7 @@ class Database:
             OpenTrigger(),
             CloseTrigger(),
             GoTrigger(),
+            HelpTrigger(),
         ]
     )
 
@@ -56,6 +57,13 @@ class Database:
     def add_trigger(self, trigger: Trigger):
         self.triggers += [trigger]
 
+    def add_pre_trigger(self, trigger: Trigger):
+        """
+        Add a trigger to the front of the trigger list. This can be used to
+        override the behavior of a trigger that is added by default.
+        """
+        self.triggers = [trigger] + self.triggers
+
 
 @dataclasses.dataclass
 class Game: