Pagedown: Improve MathJax support

[RLesur]

There are two bugs with MathJax:

1. Use of a local version of MathJax
   pagedown::html_paged(self_contained = FALSE, mathjax = "local") does not use the local version of MathJax.

2. Self contained documents

   • with self_contained = TRUE, maths are not rendered with MathJax. For now, a simple workaround is to use pandoc_args = "--mathjax" (I saw that @brentthorne uses this trick).
   • rmarkdown throws the following warning

   MathJax doesn't work with self_contained when not using the rmarkdown "default" template.
   

   see https://github.com/rstudio/pagedown/issues/128#issuecomment-518371613

[RLesur]

@brentthorne With the development version of pagedown, you no longer need to use pandoc_args="--mathjax" (there is no problem if you use it).

[brentthorne]

Great thank you!