[commands] Add special extension function 'teardown' for clean-up.

This is to support people who want to clean up some external resource
that the extension is maintaining outside of a cog, where __unload
should be used instead.

The callable is the antipode of 'setup' and takes the same sole
parameter, the bot, after all commands, events, and cogs have been
unloaded.

Fixes #405.

discord/ext/commands/bot.py

@@ -764,6 +764,16 @@ class Bot(GroupMixin, discord.Client):
             for index in reversed(remove):
                 del event_list[index]
 
+        try:
+            func = getattr(lib, 'teardown')
+        except AttributeError:
+            pass
+        else:
+            try:
+                func(bot)
+            except:
+                pass
+        finally:
             # finally remove the import..
             del lib
             del self.extensions[name]