highlight lines in code chunks

[fmichonneau]

originally suggested here: https://github.com/datacarpentry/sql-ecology-lesson/issues/231

What would it take to allow lesson authors to highlight specific lines/segments in code chunks?

[brownsarahm]

Could it be done by defining css for the <em> tag inside a code block so that lesson authors could use *highlighted part* inside the code block?

also note: I'm speculating without having looked at any css, only how jekyll converts * into html.