feat(api): at_edge may now be a user-defined function

[mrjones2014]

Fixes #85

[mrjones2014]

@IndianBoy42 Does this work for you?

[IndianBoy42]

      at_edge = function(ctx)
        if ctx.direction == "left" then
          vim.cmd.NvimTreeFocus()
        elseif ctx.direction == "right" then
          vim.cmd.Trouble()
        else
          ctx.split()
        end
      end,

This seems to work great! thanks again

I did notice that the context object doesn't have a wrap() helper, a user might want to fallback to wrapping unlike me right?

[mrjones2014]

Good call. Added.