Synopsis:
   bind <key> underline

Description:
   This function toggles the current underline setting.  Due to internal
   limitations of the client, UNDERLINE is displayed as a _ on the input line,
   regardless of the key it is bound to.

See Also:
   bind(4) blink, bold, highlight_off, reverse;
   set(4) underline_video

Other Notes:
   Underlined text can be typed at any time, but it will only be displayed
   if UNDERLINE_VIDEO is on.