api: Implement POST /messages/flags.

zulip/zulip/examples/update-flags

@@ -0,0 +1,26 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+import argparse
+
+import zulip
+
+usage = """update-flags (add | remove) <flag> <message_id1> [more_message_ids]
+
+Example: update-flags add read 4 8 15
+Example: update-flags remove starred 16 23 42
+"""
+
+parser = zulip.add_default_arguments(argparse.ArgumentParser(usage=usage))
+parser.add_argument('op', choices=['add', 'remove'])
+parser.add_argument('flag')
+parser.add_argument('messages', type=int, nargs='+')
+options = parser.parse_args()
+
+client = zulip.init_from_options(options)
+
+print(client.update_message_flags({
+    'op': options.op,
+    'flag': options.flag,
+    'messages': options.messages
+}))