Replace wkhtmltopdf with asciidoctor-web-pdf

[danyill]

See #239 for background and context. There are a pile of other issues which have been logged more recently I think that would also be resolved with this.

Although asciidoctor-web-pdf is still alpha, incorporating it now might provide valuable testing feedback and wkhtmltopdf is hardly a performant or even in a functional state for many use cases.

I've prototyped this and found it to work fine. The main downside is the need to download a fairly large binary and a slightly slower pdf output.