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.
Merge pull request #15 from cuviper/master

Map <Leader>- similar to :Sexplore
Fix jumps to the root directory.

It seems that sometimes netrw loses the current dir name and
expand('%') becomes empty string.
Using '-' in this situation will jump to the root directory.
This fix saves the original netrw's '-' mapping and uses it to
browse up through folders.