Add a command line option to run.py for bot config files.

Before this change, we were looking for config files in
default locations in source control, which is not a good
place to look for them.  Now `run.py` and friends have a
command line argument where users can specify the config
files.

Note that the change to server.py is only a partial fix
to make it so that bots that don't use third party config
files won't crash.  That program needs an overhaul, anyway.

zulip_botserver/zulip_botserver/server.py

@@ -54,7 +54,13 @@ def load_bot_handlers():
         try:
             bot_dir = os.path.join(os.path.dirname(os.path.abspath(__file__)),
                                    'bots', bot)
-            bot_handlers[bot] = ExternalBotHandler(client, bot_dir, bot_details={})
+            # TODO: Figure out how to pass in third party config info.
+            bot_handlers[bot] = ExternalBotHandler(
+                client,
+                bot_dir,
+                bot_details={},
+                bot_config_file=None
+            )
         except SystemExit:
             return BadRequest("Cannot fetch user profile for bot {}, make sure you have set up the flaskbotrc "
                               "file correctly.".format(bot))