Suport for docfile, docfilesuffix, docname attributes

[GauntletPL]

Using {docfile}, {docfilesuffix} or {docname} attributes defined in official asciidoctor manual (https://asciidoctor.org/docs/user-manual/#env-attributes) does not produce any results. At the same time {docdir} works perfectly fine.

[danyill]

We run Asciidoctor by passing the document to stdin not writing to the filesystem which is why these attributes don't appear (and so you can have a preview without saving your file)

That's not to say that we can't set them when we call the processor though, it's a fairly simple change.

[danyill]

We get docdir because we explicitly set base_dir. I'll do a PR for this.