[FR] `gitbook()` should support more highlighting configuration

[cderv]

Following a discussion in #1053 it seems that gitbook() does not default to highlight.js as html_document() even if it is based on it. gitbook() can only use Pandoc syntax highlighting for now.

There is an issue already about downlit support for auto linkink: #844

This is related as it would bring downlit parser for highligting.

This is also possible required to have a workaround for #1157