Refactor the bindings for scrolling.

  This is the initial part of #175, which will involve adding
 a new mode, and which behooves me to remove duplication.
  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
  Updated for navigation and text-display.

  The (temporary) function "x()" will show Lua-supplied text.

  Updates #175.
  Updated to show text.

  The `show_text()` method will display an array of text
 (read a table, obviously).

  Updates #175
  Style-fixups.
  Allow the help() primitive to show the manual.

  If no primitive is named then show the manual, from
 the file `lumail.help`.

  This updates #175
  Added help-text/manual for the help() primitive.
  Put the release-version in the help file.
  Updated such that "H" == Show help().
  Removed duplication.

  Moved up()/down()/page-up()/page_down() into the global
 keymap.
  Bugfix:  page-up/page-down works.
  Bound text-display