Fix for  Issue #31, clean up lua stack if a variable not found
Issue #31: fix Lua stack clean up
  get/set the user's editor via editor().

  This fixes #32.

  editor() is used, if not set to an entry then $EDITOR is used,
 if that is empty we use vim.
  Added message_filter primitive.

  This will be used for issue #33.
  Document the message_filter implementation - not coded.

  This is part if issue #33.
  message_filter is now used.

  I tested this via:

        message_filter( "cat /etc/passwd" )

  Then read a message.  Instead of the mesage body I got the password
 file :)

  Fixes #33.  Closes #33.
  Document the editor() setting.
  Removed the "TODO" reference.
  Give a stub-example of message_filter().

  We can't install one by default for obvious reasons.
  Add another example use of message_filter.

  This one converts the body to upper-case.
  Updated the unused-variable hack.

  Using this should be more portable.
  Clear the screen before redrawing the index if no messages.

  This solves the problem of limiting to new-only mesages from
 a folder of many messages - and there being no refresh.
  Added global_unread() function.

  This shows all unread messages, in all folders.  Awesome.
  Remove current_maildirs() calls.

  This function does exist, but it isn't documented and it isn't
 required for the sample code either.