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