How do I tell `pandoc.write` to produce a standalone Markdown document (with YAML metadata)? There's no `standalone` option for `pandoc.WriterOptions`. I thought setting a template  might  work,  but it doesn't: the saved file does not contain a YAML block, even though the document has metadata. My attempt is below:

```

function write_markdown_file(doc, filename)
  local options = pandoc.WriterOptions{
    template = pandoc.template.default('markdown')
  }
  local data = pandoc.write(doc, 'markdown', options)
  local file <close>, error = io.open(filename, 'w')
  if file == fail then
    io.stderr:write(string.format('Failed to write Markdown file: %s\n', error))

    os.exit(1)

  end
  file:write(data)
end
```

I'm using pandoc 2.19.2.