Export published_date (and other data model references) to asciidoc

[ge0ffrey]

This asciidoc line currently doesn't work:

This website was generated on {published_date}.

This and other properties that are ready at asciidoc parsing time should be available as adoc attributes (possibly namespaced as "jbake.").

[LightGuard]

Does this relate to #467?

[jonbullock]

Hmm asciidoctor.attributes.export being set to true would expose all configuration values (specifically config.[option]) to Asciidoc but it shouldn't expose any of the JBake data model unfortunately.