Enable and then add (or replace) line highlighting for code syntax

[RichieEscarez]

https://gohugo.io/content-management/syntax-highlighting/#highlighting-in-code-fences

With Hugo 0.60.x you can specify which lines in a code block that you want highlighted (for example to show/demonstrate how and where to add an element in a config file).

If you add hl_lines (ie. hl_lines=8 15-17), the specified line numbers get yellow highlighting.

Nice to have for our readfile shortcode