Add support for adoc and ad extensions

.adoc and .ad have become common AsciiDoc file extensions. For example they are supported by Sublime, github, AsciiDoctor Live Preview and the referenced in the AsciiDoctor Writers Guide, etc.

https://github.com/SublimeText/AsciiDoc/pull/4
https://github.com/github/markup/pull/88
https://github.com/asciidoctor/asciidoctor-chrome-extension/blob/master/README.adoc
http://asciidoctor.org/docs/asciidoc-writers-guide/