Add attributes to asciidoc

- :partnums: Book parts are assigned roman numerals when the attribute partnums is set.
- :sectnums: If set, numbers sections to depth specified by sectnumlevels(default 3).
- :sectlinks: Turns section titles into self-referencing links.
- :!compat-mode: use the new Asciidoctor syntax

book.asciidoc

@@ -6,6 +6,10 @@
 :toclevels: 3
 :experimental:
 :icons: font
+:partnums:
+:sectnums:
+:sectlinks:
+:!compat-mode:
 
 include::chapters/preface.asciidoc[]