Asciidoctor: support include directives for other asciidoc files

A lot of time has passed since latest comments from devs in #172 and #335.
I think after years things have changed in your infrastructure, why not to bring up this issue again with some more input and news?

For example, AFAIK GitHub started to use [containers](https://githubengineering.com/kubernetes-at-github/) which could help to isolate things from unintentional access and improve security.

Also take a look at asciidoctor/asciidoctor#1088. The author of Asciidoctor is [ready for a conversation](https://github.com/asciidoctor/asciidoctor/issues/1088#issuecomment-323672651) about this, there is even proposed a way to implement this with [custom include preprocessor](https://github.com/asciidoctor/asciidoctor/issues/1088#issuecomment-87685739). If there are still some concerns about resolving the issue, please provide a constructive feedback so both sides can collaborate.