AsciidoctorJ Screnshot Extension

Hi

I created a standalone screenshot extension based on the work of François-Xavier Thoorens. He discussed his work on this list: screenshot-plugin-for-asciidoctor

As a first version I did not incorporate most of the suggested changes into the code.
If you still find it useful I would donate this to the asciidoctor community and we can create issues on github for the suggested improvements.

Cheers
Stephan

asciidoctorj-screenshot
asciidoctorj-sreenshot-example