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.

先閱讀 #172 和 #335 中的歷史記錄，然後檢視 asciidoctor/asciidoctor#1088 中提出的自訂 include 前處理器。在確定實作路徑之前，先與維護者釐清安全性和基礎設施方面的限制。完成標準是能夠安全地支援 include 其他 AsciiDoc 檔案，同時不會產生非預期的存取。

由索引模型根據 Issue 內容生成。