Thanks for the great package!

Custom URLs for MathJax aren't working for me in Bookdown. I have tried setting the mathjax option in _output.yml as below. However the rendered HTML just uses the out of date version 2.7.2 of MathJax from mathjax.rstudio.com/latest, not the version I have specified.

Am I doing something wrong, or is this a bug?

I can see in RStudio that the mathjax variable argument has been passed to the pandoc command line call. However it seems that the variable isn't being processed into the template gitbook.html.

I asked asked about this on StackOverflow, but I got no response. [Edit: fixed link]

Minimal example:

_output.yml:

bookdown::gitbook:
  css: style.css
  mathjax: "https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js" 

index.Rmd:

--- 
title: "Untitled"
author: "An author"
site: bookdown::bookdown_site
documentclass: book
---

$$E=mc^2$$

Session info:

> xfun::session_info('rmarkdown')
R version 3.6.3 (2020-02-29)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS Mojave 10.14.6, RStudio 1.3.959

Locale: en_GB.UTF-8 / en_GB.UTF-8 / en_GB.UTF-8 / C / en_GB.UTF-8 / en_GB.UTF-8

Package version:
  base64enc_0.1.3 digest_0.6.25   evaluate_0.14  
  glue_1.4.1      graphics_3.6.3  grDevices_3.6.3
  highr_0.8       htmltools_0.5.0 jsonlite_1.7.0 
  knitr_1.29      magrittr_1.5    markdown_1.1   
  methods_3.6.3   mime_0.9        rlang_0.4.7    
  rmarkdown_2.3   stats_3.6.3     stringi_1.4.6  
  stringr_1.4.0   tinytex_0.24    tools_3.6.3    
  utils_3.6.3     xfun_0.15       yaml_2.2.1     

Pandoc version: 2.9.2.1

By filing an issue to this repo, I promise that

• I have fully read the issue guide at https://yihui.org/issue/.
• I have provided the necessary information about my issue.
  • If I'm asking a question, I have already asked it on Stack Overflow or RStudio Community, waited for at least 24 hours, and included a link to my question there.
  • If I'm filing a bug report, I have included a minimal, self-contained, and reproducible example, and have also included xfun::session_info('bookdown'). I have upgraded all my packages to their latest versions (e.g., R, RStudio, and R packages), and also tried the development version: remotes::install_github('rstudio/bookdown').
  • If I have posted the same issue elsewhere, I have also mentioned it in this issue.
• I have learned the Github Markdown syntax, and formatted my issue correctly.

I understand that my issue may be closed if I don't fulfill my promises.