Start to make colours more flexible.

  Rather than just definining "red on black" and "white on black"
 we've now setup a simple colour map.  We use the map to lookup
 the colour attributes.

  The end result is that we're still showing new folders/messages
 in red, but we can now change them to be blue, green, or similar
 easily.

  TODO:  Expose colour settings to lua.