bots: Add manual command to get bot output.

This eliminates the need to setup dev environment and to
create a bot, setup zuliprc file, subscribe the bot to the
stream in order to try out a bot.

Manual command get-bot-output gives the bots response content
directly.

zulip_bots/setup.py

@@ -27,6 +27,7 @@ package_info = dict(
     entry_points={
         'console_scripts': [
             'zulip-run-bot=zulip_bots.run:main',
+            'zulip-bot-output=zulip_bots.zulip_bot_output:main'
         ],
     },
     include_package_data=True,