Support for appendix section numbering

[xcoulon]

For example, in a document such as:

= Article Title
:appendix-caption: Exhibit
:sectnums:
:toc:

== Section

=== Subsection

[appendix]
== First Appendix

=== First Subsection

=== Second Subsection

[appendix]
== Second Appendix

appendix sections will be numbered Exhibit A, A.1, A.2, etc.

see https://docs.asciidoctor.org/asciidoc/latest/sections/appendix/#appendix-section-syntax