Asciidoctor core now uses read_contents to download and embed custom stylesheet

[ggrossetie]

Adjust the implementation to be compatible with asciidoctor/asciidoctor#3766

[mojavelinux]

Long term, we really need to think about how the definition for what a remote URL is can be controlled. For example, even though chrome:// looks like a remote URL to core, it's actually a specialized file URI for the browser extension. We shouldn't be circumventing allow-uri-read, but changing when allow-uri-read is requested. And that is only once we know, for certain, that the target requires a network call.

[ggrossetie]

@mojavelinux Yes!
In this case, we should probably clarify the intent of allow-uri-read:

For security reasons, this capability is not enabled by default. To allow content to be read from a URI, you must enable the URI read permission by:

-- gh.com/asciidoctor/asciidoctor.org/master/docs/_includes/uri-include.adoc

It should be: "To allow content to be read from a remote URI..."

(And we want enough logic hooks for that).

I think a is_remote? check/hook in core would help but since I'm already monkey-patching the whole read_contents there's no rush.