Add <Plug> nmaps for split windows

Extract the meat of <Plug>VinegarUp into function s:opendir(), with an
argument for the command to use, at first 'edit'.  Then add variants
<Plug>VinegarSplitUp and <Plug>VinegarVerticalSplitUp which use 'split'
and 'vsplit' respectively.  Users may define their own maps which
exercise these choices.