Use the html4 output format instead of the default html5 in Pandoc 2.0

[yihui]

Fixes #149.

Basically Pandoc 2.0 starts to default to html5 when the output format is the ambiguous html (previously it was html4), and the consequence is that the section divs actually use the <section> tag instead of <div>. See https://groups.google.com/forum/#!topic/pandoc-discuss/d-d07z2iHJs for more information.

I have compared the output with this PR + Pandoc 2.0 against Pandoc 1.19, and the output files seem to be identical.

[jjallaire]

That's excellent! Thanks for sorting that out :-)

@bborgesr Let's plan on a CRAN release for flexdashboard as well. We should also take a look at learnr to see if we need the same fix there and then also submit that to CRAN.

[bborgesr]

@jjallaire: in learnr, I think there's just one instance with this problem: https://github.com/rstudio/learnr/blob/master/R/tutorial-format.R#L101-L103

It's simple enough to change it there, but at least the two demo tutorials ran fine for me with the newer version of pandoc and no changes. I'm not familiar enough with the package to know if that's representative. I'd err on the side of caution and substitute those lines in learnr a la this PR. Even if we're not preventing any bugs, at least, learnr would be internally consistent with all of these other rmarkdown/pandoc packages.

I can go ahead with that if you agree.

As far as the releases go, is there a release checklist that I can go through? I can improvise if not; I just wanted to make sure I don't have any blind spots.

[jjallaire]

Yes, let's change it there just to be on the safe side.

No release checklist. My guess is that there are very very few changes from the last release so just doing a global diff and then scenario and/or unit testing as appropriate around the diff will be fine.