[commands] Add support for stacking Cog.listener decorator.

Fix #1926

discord/ext/commands/cog.py

@@ -102,7 +102,8 @@ class CogMeta(type):
                 except AttributeError:
                     continue
                 else:
-                    listeners.append((value.__cog_listener_name__, value.__name__))
+                    for name in value.__cog_listener_names__:
+                        listeners.append((name, value.__name__))
 
         attrs['__cog_commands__'] = commands # this will be copied in Cog.__new__
         attrs['__cog_listeners__'] = tuple(listeners)
@@ -209,7 +210,11 @@ class Cog(metaclass=CogMeta):
             if not inspect.iscoroutinefunction(func):
                 raise TypeError('Listener function must be a coroutine function.')
             func.__cog_listener__ = True
-            func.__cog_listener_name__ = name or func.__name__
+            to_assign = name or func.__name__
+            try:
+                func.__cog_listener_names__.append(to_assign)
+            except AttributeError:
+                func.__cog_listener_names__ = [to_assign]
             return func
         return decorator