textproc/rubygem-asciidoctor-epub3: New port

Convert AsciiDoc documents to EPUB3

Asciidoctor EPUB3 is a set of Asciidoctor extensions for converting AsciiDoc
documents directly to the EPUB3 and KF8/MOBI e-book formats. It's a tool for
creating highly aesthetic, professional, easy-to-read e-books.

WWW: https://github.com/asciidoctor/asciidoctor-epub3