Make the hipchat background colour configurable

The HipChat [docs](https://www.hipchat.com/docs/api/method/rooms/message) state that the default is yellow. 

Not quite with on the space issue. This is the current line:

    settings[:color] = 'yellow' unless COLORS.include? settings[:color].to_s

Do you think it would be clearer as:

    settings[:color] = 'yellow' unless COLORS.include?(settings[:color].to_s)

?