additional work on dzslides backend in slim

- note that these templates rely on the following dzslides fork
https://github.com/mojavelinux/dzslides
rename some options to align w/ AsciiDoc conventions, update docs

- incremental="true" changed to options="step"
- fullbackgroundimage changed to canvas-image
- fullbackgroundtext changed to role="canvas-text"
small addition to doc
Merge branch 'master' of github.com:asciidoctor/asciidoctor-backends