Margin reference update

[eddelbuettel]

@cderv tweeted

so may need to update here too.

[cderv]

Yes I think you'll also need to fix https://github.com/rstudio/tufte/issues/91:

With Pandoc 2.11+

---
title: "Margin references"
output:
  tint::tintHtml: default
bibliography: test.bib
link-citations: yes
---

```{r}
knitr::write_bib(c("knitr", "tint"), "test.bib")
```

See @R-tint and @R-knitr