Correctly move references in margin with Pandoc 2.11 new citeproc

[cderv]

This will close #85

First change is to take into account the new classes added by Pandoc. Ex:

<div id="ref-xie2015" class="csl-entry">
Xie, Yihui. 2015. <em>Dynamic Documents with <span>R</span> and Knitr</em>. 2nd ed. Boca Raton, Florida: Chapman; Hall/CRC. <a href="http://yihui.name/knitr/">http://yihui.name/knitr/</a>.
</div>

Second change is that new citeproc will generate two links, e.g Allaire et al. (2020)

<span class="citation"><a href="#ref-R-rmarkdown" role="doc-biblioref">Allaire et al.</a> (<a href="#ref-R-rmarkdown" role="doc-biblioref">2020</a>)</span>

Previously with Pandoc 2.7, we had only one, e.g Allaire et al. (2020)

<span class="citation">Allaire et al. (<a href="#ref-R-rmarkdown" role="doc-biblioref">2020</a>)</span>

And tufte seems to support from a long time now, merging links (d6a5748) - it seems to be for 2016a, so year with suffix.

@yihui, I tried to follow the same logic with regex (and it is a use case for regexec maybe?).
However, I wonder if we should not have a condition on rmarkdown::pandoc_available("2.11") to have a new regex for new citeproc. Two processing for this, depending on the Pandoc version.
In fact, I am not sure if there won't be more than 2 links sometimes. How do I generate a reference entry with a alphatetic suffix to try with Pandoc 2.11 ?

Also, with this PR, the linked will be merged and, with Pandoc 2.11+, we will have the toggle logic to apply on the all Name + year (Allaire et al. (2020)) and not just the year as before.

With Pandoc 2.11:

With Pandoc 2.7.3:

I left this PR as a draft as I am not sure of the fix and this need to be tested with alphabetic suffix

[cderv]

How do I generate a reference entry with a alphatetic suffix to try with Pandoc 2.11 ?

In fact, this is when two entries have the authors and years. I tested ny adding flextable and officer in knitr::write_bib(c('base', 'rmarkdown', 'flextable', 'officer'), file = 'skeleton.bib')

then referencing them in the text. It works ok

This is because new citeproc will not generate two links, it will be (Gohel 2020a) before tufte processing

Note that with Pandoc 2.11, [@R-flextable] will generate one link between parenthesis, and @R-flextable will generate two links, one for the author, and one for the year, parenthesis around the year not being included.

[cderv]

And on the contrary of what I did for bookdown, the classes added by Pandoc to style the references div is not kept here. Are you fine with that ?

I think it is ok because tufte is a very special styled format.

[cderv]

I tested that this both works for Pandoc 2.7.3 where
Gohel (2020a) needs to be merge in one 2020a and with Pandoc 2.11 where Allaire et al. (2020) needs to be merge also in one Allaire et al. (2020)

[yihui]

Looks good to me. Thank you!