Improve :menu and :menutranslate highlighting.
- Match args to :menutranslation and :popup.
- Only highlight special notation in {rhs} of :menu, like :map.
- Allow line continuations in {rhs} of :menu and between {english} and
{mylang} of :menutranslation, matching common usage.
- Bug fixes.
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
