Correctly deactivate math support in Rmd for @includeRmd with math

[cderv]

This follows up on #1304 to fix rstudio/rmarkdown#2331

This will now correctly modify option to rmarkdown::github_document() conditionnaly. I messed up my test last time but now this work as expected.

Tested by:

• Cloning this project: https://github.com/EmilHvitfeldt/codecogsreprex
• Installing this PR
• running devtools::document()

This produce what is explained in the comment in previous PR: #1304 (comment)

This in markdown

$$
\eta_{i} = (\beta_0 + b_{0i}) + \beta_1x_{i1}
$$ 

will produce this in Rd file

\emph{η}\if{html}{\out{<sub>}}\emph{i}\if{html}{\out{</sub>}} = (\emph{β}\if{html}{\out{<sub>}}0\if{html}{\out{</sub>}}+\emph{b}\if{html}{\out{<sub>}}0\emph{i}\if{html}{\out{</sub>}}) + \emph{β}\if{html}{\out{<sub>}}1\if{html}{\out{</sub>}}\emph{x}\if{html}{\out{<sub>}}\emph{i}1\if{html}{\out{</sub>}}

as Pandoc will convert this math using Plain text. Complex LaTeX math won't work in github document output as pandoc will produce this

$ echo "$\frac{1}{2}$" | pandoc -t gfm
[WARNING] Could not convert TeX math \frac{1}{2}, rendering as TeX
$\\frac{1}{2}$

@EmilHvitfeldt can you confirm this time this PR work on your projects ? Thank you !

@hadley I am trying to add test so that change on math in included Rmd can be detected

[EmilHvitfeldt]

This works for me test in reprex and parsnip 🎉

[cderv]

This could also be done probably by creating the format with option using a do.call(rmarkdown::github_document, ...)
I can change if that better fits the usual practice or if it seems clearer.