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 pull request #2 from mojavelinux/deckjs-improvements

Rename some options to align w/ AsciiDoc conventions, update docs
additional work on dzslides backend in slim

- NOTE: dzslides backend depends on this fork of dzslides
  https://github.com/mojavelinux/dzslides