Improve :echo and :execute highlighting.
- Add better line-continuation support for both commands.
- Improve the :execute command's expression argument matching.
- Remove the fix for issue #9987 as this is now handled by correctly
matching the parens in :echo (...) as operator parens.
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
