Have the bot use the correct prefix in examples, even if it's hard coded

Basically this just looks at the ```@example``` decorator's text to ensure that if the prefix set in config is different or missing, it gets correctly prepended to the help text.

My users were very confused when I changed the prefix to @ but the help commands still said to use "." :)

Also, I *love* this bot.  Switching from Hubot to Willie was a huge time-saver for me!