Adds twemoji integration

You have an extra trailing double quote here.

Also, this uses a full file system path. Could you copy the stylesheet to the output directory (if it's different than the current directory) and use a relative link? If you want to go the extra mile, you'd honor the stylesdir and the copycss settings as you see here: https://github.com/asciidoctor/asciidoctor/blob/master/lib/asciidoctor.rb#L1521

Keep in mind that you can get the output directory in Asciidoctor 1.5.2 using the "outdir" attribute. It's not always available, so you probably want to fallback to "docdir" (even if it might be wrong).