Asciidoctor extensions port created

[nicorikken]

Thanks @amsardesai for creating this repo. I just reused it in creating an extension for the Asciidoctor project: asciidoctor/asciidoctor-extensions-lab#70 Although I could introduce Middleman as a dependency and use this complete repo, I decided a port was nicer and pretty straightforward as well.

I noticed there are 2 known deficiencies (internationalization and non-link replacement). If I get around to improving on theses issues, I intend to contribute back upstream.

If you have any remarks on my port, please let me know.