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.
  Fixed the definition of the colour "red".
  Added new primitives: unread_maildir_colour + unread_message_colour.

  These two primitives will allow you to change the default behaviour
 of displaying messages which are new/unread, and folders which contain
 new/unread mails, in red.

  You can choose from "blue", "cyan", "green", "magenta", "red",
 "white", & "yellow".

  Documentation is now complete too:

    http://lumail.org/lua/unread_maildir_colour.html
    http://lumail.org/lua/unread_message_colour.html
  New primitive: attachment_colour().

  This colour is used to draw the list of attachment-names
 beneath the message header of any mail that contains attachments.
  Added new primitives: header_colour() + body_colour().

  These control the colour of text in message-display.

  This completes the initial pass at configurable colour
 support.
  Updated changelog for debian package