RFE: watch(1) option to clear the display between command invocations

When running watch, if there are commands in background shell jobs generating output, the display gets corrupted, which persists across updates.

The RFE here is a non-default option that clears the display before pasting the command output, likely a single terminfo call. Since this may make the display a bit flickery it should not be on by default.

I'd implement this myself if I had the coding chops :-/