plugings/obsidian: add follow_img_func function setting

[jonboh]

This setting is almost identical to follow_url_func.
Currently if one tries to add this setting like this:

plugins.obsidian = {
  # ...
  settings = {
    follow_img_func = ''
    function(img)
      vim.fn.jobstart { "xdg-open", img }
    end
  ''; 
    follow_url_func = ''
      function(url)
        vim.fn.jobstart({"xdg-open", url})  -- linux
      end
  '';
}; 
}    

The generated configuration uses the literal string, and fails when used:

      follow_img_func = 'function(img)\n  vim.fn.jobstart({"xdg-open", img})  -- linux\nend\n',
      follow_url_func = function(url)
          vim.fn.jobstart({ "xdg-open", url }) -- linux
      end,