Map <Leader>- similar to :Sexplore

Extract the meat of `<Plug>VinegarUp` into function s:opendir(), with an argument for the command to use, at first 'edit'.  Then add a variant `<Plug>VinegarSplitUp` which uses 'split' instead, and map to `<Leader>-`.

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If you don't like `<Leader>-` added by default, please consider still adding `<Plug>VinegarSplitUp` so users can choose some mapping for this.