Removed screenshots.

  They are visible upon the lumail website:

        http://lumail.org/screenshots/
  Added save_message().

  This allows a currently-selected message to be saved to a new
 location.

  TODO:
   * Avoid using /bin/cp
   * Create destination if missing.
  Added history to the input function prompt().

  The history was previously one entry "deep".  Now we keep track
 of all history when the program runs, although this isn't saved
 to disk.

  This updates issue #5.
  Added comments to functions.
  Renamed member variable.

      m_map -> m_history

  This is was a historical thing; I previously stored history in
 a map but later changed to a vector.
  Prompting for a maildir works.
  Updated the comments considerably.

  The file was previously very readable, but it is now 100%
 current and correct.  With minor clarifications added for
 neatness.
  Ctrl-l also redraws the display.

  Some programs use ctrl-r some use ctrl-l.  We should allow both
 by default to avoid surprises.