Updated logic for message-display.

  This simplifies things as part of #194.

  TODO:  Split headers on newline/comma and allow them to be
 bundled.

  e.g. This:
     To: foo@bar.com, example@example.com, steve@steve.com

  should become:

    To: foo@bar.com,
     example@example.com,
     steve@steve.com
  Wrapping works for headers now too.

  OK this was easier than expected, but there might be cases
 where it doesn't work fully.  Updates welcome.

  This updates #194 - probably closes it, but I'll wait and see.

  (Obviously you must enable `wrap`.)
  Update the length-tracking of wrapping-headers.

  Ths updates #194.
  Minor update relating to line-wrapping.

  Rename the toggle_line_wrapping function to be clearer,
 after initially just fixing a comment-typo.