consider --standalone output

[tjmahr]

According to the pandoc documentation, yaml metadata is only written to markdown output if --standalone is used:

When pandoc is used with -t markdown to create a Markdown document, a YAML metadata block will be produced only if the -s/--standalone option is used. All of the metadata will appear in a single block at the beginning of the document.

As a result, downlit::downlit_md_path() discards YAML metadata when run on markdown files. This prevents me from performing markdown -> downlit -> markdown conversions.

The result of setting --standalone is:

When the -s/--standalone option is used, pandoc uses a template to add header and footer material that is needed for a self-standing document. To see the default template that is used, just type pandoc -D *FORMAT*

If I call system2(rmarkdown::pandoc_exec(), "-D gfm") or system2(rmarkdown::pandoc_exec(), "-D markdown_github"), I get a barebones template which should not otherwise affect the md output.

$if(titleblock)$
$titleblock$

$endif$
$for(header-includes)$
$header-includes$

$endfor$
$for(include-before)$
$include-before$

$endfor$
$if(toc)$
$table-of-contents$

$endif$
$body$
$for(include-after)$

$include-after$
$endfor$