Add option to not automatically join rooms on invite

I couldn't follow the same convention for setting the value of HUBOT_HIPCHAT_JOIN_ROOMS_ON_INVITE on options because

    joins_room_on_invite: process.env.HUBOT_HIPCHAT_JOIN_ROOMS_ON_INVITE or true 

would always resolve to true.  Let me know if you've got a preferred way of handling this.  I'm certainly no CoffeeScript expert.