Add support for the AsciiDoc syntax

The only Java lib I know that parses AsciiDoc is AsciiDoctor but it has some issues:

• It's written in Ruby (which means slow startup time, slow execution time and large memory footprint)
• It doesn't seem to have Events yet. See:
  • http://discuss.asciidoctor.org/Events-in-the-Java-integration-tc1190.html
  • http://discuss.asciidoctor.org/Is-there-any-AST-or-events-exposed-by-asciidoctor-parser-tc62.html
  • https://github.com/asciidoctor/asciidoctor/issues/60
  • http://discuss.asciidoctor.org/Custom-backend-how-easy-tc1151.html#a1154