[commands] Allow customising the Context class in get_context.

discord/ext/commands/bot.py

@@ -555,7 +555,7 @@ class BotBase(GroupMixin):
             return prefix
 
     @asyncio.coroutine
-    def get_context(self, message):
+    def get_context(self, message, *, cls=Context):
         """|coro|
 
         Returns the invocation context from the message.
@@ -572,15 +572,21 @@ class BotBase(GroupMixin):
         -----------
         message: :class:`discord.Message`
             The message to get the invocation context from.
+        cls: type
+            The factory class that will be used to create the context.
+            By default, this is :class:`Context`. Should a custom
+            class be provided, it must be similar enough to :class:`Context`\'s
+            interface.
 
         Returns
         --------
         :class:`Context`
-            The invocation context.
+            The invocation context. The type of this can change via the
+            ``cls`` parameter.
         """
 
         view = StringView(message.content)
-        ctx = Context(prefix=None, view=view, bot=self, message=message)
+        ctx = cls(prefix=None, view=view, bot=self, message=message)
 
         if self._skip_check(message.author.id, self.user.id):
             return ctx