resolves #160 add support for defining global AsciiDoc attributes in the playbook or using a CLI option
- add asciidoc category to playbook schema
- add attributes key to asciidoc category of playbook schema
- accepts attributes defined as an object (key/value pairs)
- add test to playbook builder to test asciidoc.attributes value
- add custom object format to convict for parsing key/value pairs from string
- interpret values as YAML data values
- pass playbook.asciidoc.attributes to convertDocument
- pass playbook.asciidoc.attributes to buildNavigation
- add argument to buildNavigation function to accept attributes