feat: accept a function that returns the layout to use

[ouuan]

This resolves #1080.

Example:

  window = {
    layout = function()
      if vim.o.columns > 2 * vim.o.lines then
        return 'vertical'
      else
        return 'horizontal'
      end
    end
  },

[deathbeam]

Thanks!

@all-contributors add @ouuan for docs and code

[allcontributors]

@deathbeam

I've put up a pull request to add @ouuan! 🎉