[commands] Allow converters from custom discord.ext extensions

Originally the converting of an argument with type in the discord.*
package would try to use the predefined converters, even if there were
none present. This is fixed by supplying a default argument to getattr

Fixes #2369

discord/ext/commands/core.py

@@ -347,7 +347,7 @@ class Command(_BaseCommand):
             pass
         else:
             if module is not None and (module.startswith('discord.') and not module.endswith('converter')):
-                converter = getattr(converters, converter.__name__ + 'Converter')
+                converter = getattr(converters, converter.__name__ + 'Converter', converter)
 
         try:
             if inspect.isclass(converter):