use Asciidoctor to render AsciiDoc by default

- allow the site config asciidoc.engine to control which engine is used
- set default engine to asciidoctor
- load required library lazily
- add test to ensure asciidoctor is loaded properly
- fix the call to the asciidoc system command (but leave test disabled)
Merge pull request #197 from mojavelinux/asciidoctor

Use Asciidoctor to render AsciiDoc by default