Add an option to opt-out using header-attrs HTML dependency in html_document_base()

[cderv]

Currently we use a JS script in a custom html_dependency_header_attrs() so that we can "fix" breaking pandoc behavior from change in 2.8. We do that when Pandoc 2.9+ is used.

With rmarkdown we currently use pandoc's --section-div flag by default. This will create a <div> to encapsulate header. When attributes are added on header, they are moved to this section div. Behavior changed in Pandoc 2.8

• Text to convert using pandoc --to html4 --section-div

  # hello {.anchor #id attr='val'}

• With Pandoc 2.7.3 - nothing added on header itself - all move to section div

  <div id="id" class="section level1 anchor" lang="en">
  <h1>hello</h1>
  </div>

• With Pandoc 2.8 - breaking change - class is not moved anymore

  <div id="id" class="section level1">
  <h1 class="anchor" lang="en">hello</h1>
  </div>

• With Pandoc 2.9 to 2.13 - only id is moved now.

  <div id="id" class="section level1 anchor" lang="en">
  <h1 class="anchor" lang="en">hello</h1>
  </div>

The change of Pandoc regarding class was breaking some tools (#1723, #1732) and so it was added a JS script to remove the class on header.

This could no be desired always like rstudio/bookdown#865 and r-lib/pkgdown#1538.

So this PR adds an option to opt-out including the HTML dependency. This would solve the two issue above IMO

[yihui]

Looks good! Thanks!