markdown -> LaTeX / PDF, use pygmentex instead of listing

[evan0greenup]

When converting markdown files with code blocks or inline code terms into PDF or LaTeX files, the current code highlight is not beautiful or expressive.

For use who install latexextra, it would be good to use pygmentex to make the code block very nice.

https://ctan.org/pkg/pygmentex?lang=en

[jgm]

We try not to depend on things in latexextra.

[jgm]

Nor would we want to depend on pygments being installed.

Note: the highlighting built into pandoc is configurable. See --highlight-style in the manual. You can even write your own styles.