Respect preview output format

[ElianHugh]

Closes #779

At the moment, previewing R Markdown documents forces the rmarkdown::html_document() format. However, this overwrites all output formats, even if they can result in a .html file.

This PR uses the object pandoc$to from rmarkdown:::create_output_format to check if the document's output format == html. This way, we only override the output format if pandoc$to does not equal html.

Screenshot

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The following renders incorrectly on master, which is fixed in this PR:

---
title: "R Notebook"
output:
  bookdown::html_document2: default
---

```{r table1}
knitr::kable(mtcars[1:5, 1:5], caption = "The mtcars data.")
```

`mtcar` data is shown in Table \@ref(tab:table1).

[renkun-ken]

The rendering works now.

But it seems the link to table 1 does not work: it will open in an external browser and go to a non-working URL.

[ElianHugh]

Thanks for picking that up - it looks like this is something to do with how webviews convert links, as the anchors work in external browsers.

Edit:
Specifically, anchor tags like #tab:table1 are converted to a webview resource scheme

[renkun-ken]

The anchor tags work nicely now.

[renkun-ken]

LGTM