Instead of requiring a bunch of `<span>` wrangling, add a custom tag,
`<Listing>`, whose attributes get rewritten into the desired output
format, using `<figure>` and `<figcaption>` for improved semantic HTML.
Support both a default (nice and fancy) HTML output mode and a “simple”
mode to make it so we get the desired output format for the NoStarch
input files as well.
