BUGFIX: Refuse to set global_mode to an unknown mode.

  We'll return an error if you try to run:

        global_mode( "cookie" );

  The valid modes are: maildir, index, message & text.
  Updated such that show_text honours colours.

  This matches the (stalled) work that has already been done,
 and allows this to work as expected:

    function x()
       txt = { "${colour:red}Steve",
               "${colour:blue}Kemp",
               "${bold}Has",
               "${underline}Definitely",
               "Made this work" }
       show_text( txt )
       global_mode( "text")
    end

  This updates #175.
  Updated the comments on formatting more.

  Moved the tokenization into a separate file.
  Added utility class.
  Merge open_selected_folder and open.

  They are now one function which will work with/without a
 folder-name as an argument.
  Bump copyright year to 2014.

  For all files modified this year.