Add AsciiDoc syntax rules

This MR adds support for highlighting the AsciiDoc mark-up language.

This is especially useful to us in Red Hat Customer Content Services (CCS) because we use AsciiDoc to write the majority of customer-facing documentation. Having highlighting support in GEdit and other editors would be helpful.

As a disclaimer, I'm not the author of the file. I've taken it from the asciidoc-highlight repository by Eduardo Santana: https://github.com/edusantana/asciidoc-highlight

The file is licensed under GNU LGPL 2 or later, so I hope this is fine. I'll notify the author for awareness.