resolves #290 upgrade to Asciidoctor.js 1.5.9
- upgrade Asciidoctor.js to 1.5.7, then 1.5.9
- remove chai object inspection workarounds
- pass partial-option to push_include in include processor
- change how xrefs are processed to be compatible with Asciidoctor 1.5.7
- change docname to use module-relative path without extension
- use the correct property to free extension instances
- add additional tests