htmlPreserve() wraps input <p> tag in some cases

[cpsievert]

Minimal reprex of the problem behind rstudio/flexdashboard#379

This issue was introduced when we added the htmltools.preserve.raw in #191, which rmarkdown now sets to preserve HTML via pandoc's raw attributes rather than our using own hand-rolled solution.

What happens is essentially:

withr::with_options(
  list(htmltools.preserve.raw = TRUE), 
  htmlPreserve("<iframe></iframe>")
)
#> [1] "`<iframe></iframe>`{=html}"

And since this is using an inline code block, pandoc wraps the HTML in a <p>, which can have numerous unintended consequences, including changing the computed height on the input HTML (as we're seeing in rstudio/flexdashboard#379)

# inline form
❯ pandoc -t html
`<iframe></iframe>`{=html}
^Z
<p><iframe></iframe></p>

# block form
❯ pandoc -t html
```{=html}
<iframe></iframe>
```
^Z
<iframe></iframe>

After running this through @jjallaire, it seems like the best way forward is to just not use the inline form at all since:

1. It doesn't seem like there's any real use case for wrapping the input in <p> (and if there is, it seems easy enough to add it to the input?)
2. This should generally yield the same result as using options(htmltools.preserve.raw = TRUE), which has been around much longer

cc @cderv