feat: Always cache remote content by default

[jboxman]

This is possible.

https://docs.asciidoctor.org/asciidoc/latest/directives/include-uri/#caching-uri-content

Reading content from a URI is obviously much slower than reading it from a local file.

Asciidoctor provides a way for the content read from a URI to be cached, which is highly recommended.

To enable the built-in cache, you must:

1. Install the open-uri-cached gem.

2. Set the cache-uri attribute in the document.

When these two conditions are satisfied, Asciidoctor caches content read from a URI according the to HTTP caching recommendations.

And these remote calls might be why in some rare situations, ascii_binder hangs forever.