First pass at preliminary thread support

This is missing a lot of functionality right now, such as two gateway
events and all the HTTP CRUD endpoints.

discord/flags.py

@@ -279,6 +279,13 @@ class MessageFlags(BaseFlags):
         """
         return 16
 
+    @flag_value
+    def has_thread(self):
+        """:class:`bool`: Returns ``True`` if the source message is associated with a thread.
+
+        .. versionadded:: 2.0
+        """
+        return 32
 
 @fill_with_flags()
 class PublicUserFlags(BaseFlags):