Allow people to use the {pandoc} R package with {sandpaper}

[zkamvar]

The {pandoc} R package is a wonderful addition that's allowed me to test different versions of pandoc... but I've run into a problem while testing #454 where I've been unable to activate the correct version of pandoc. It turns out that, because I run the pandoc conversions through callr, that uses the default system pandoc:

sandpaper/R/render_html.R

Lines 59 to 60 in e13dd0b

	callr::r(function(...) rmarkdown::pandoc_convert(...), args = args,
	show = !quiet, spinner = sho)

I would love to be able to take {pandoc} as a dependency, but the problem is that it requires a GitHub PAT to be set up to connect to GitHub to download releases.

I might be able to get around this by passing the results from rmarkdown::find_pandoc() into the callr function so that it can set the right pandoc version before running.