Unsupported messages handling

We use Lita extensively at Swiftype and really like it. The only major problem we have noticed is that when we make a typo in a command, the bot just ignores it and many users get confused assuming that the bot is taking its time to respond, while the bot just drops the message on the floor.

This pull request adds (optional) support for handling of unsupported messages. when `config.robot.handle_unsupported_messages` is set to `true` and a message is not supported by any of the registered handlers, the bot would respond with an "I do not understand" kind of message (only happens for command messages so any random messages in the chat would not trigger the new code path).