Updated to allow simple text-display.

  This update provides a new global-mode "text", with the ability
 to read/display a file.  The support is basic for now, and requires
 additional work to complete.

  To test it out run:

            view_file( "/etc/passwd" );
            global_mode( "text" );

  You can scroll up/down via "j/k/up/down", thanks to the reworked
 `up` and `down` functions defined in `lumail.lua`.

  This updates #175