followup bot: Fix help response and configure streams.

From @showell:

We had a PR here with lots going on, and the commits weren't
very well organized, and then there were some tricky merge
conflicts from another PR.  So I just squashed them all into
one commit.

What this does:
    * allow you to configure your followup stream
    * provide help in followup stream
    * add more testing to followup stream
    * add get_response() helper for tests

Fixes #173
Fixes #174

zulip_bots/zulip_bots/test_lib.py

@@ -100,6 +100,18 @@ class StubBotTestCase(TestCase):
 
     bot_name = ''
 
+    def get_response(self, message):
+        # type: (Dict[str, Any]) -> Dict[str, Any]
+        bot = get_bot_message_handler(self.bot_name)
+        bot_handler = StubBotHandler()
+
+        if hasattr(bot, 'initialize'):
+            bot.initialize(bot_handler)
+
+        bot_handler.reset_transcript()
+        bot.handle_message(message, bot_handler)
+        return bot_handler.unique_response()
+
     def verify_reply(self, request, response):
         # type: (str, str) -> None