Number appendix subsections based on appendix letter

[mojavelinux]

Numbers for appendix subsections should be based on the appendix letter (e.g., A.1, A.2).

[mojavelinux]

This was a problem specifically with the book doctype when the document was written as follows:

= Book Title
:doctype: book
:sectnums:

== Chapter

content

[appendix]
= Appendix

content

=== Appendix Subsection

content

The problem was with the model in core in Asciidoctor <= 1.5.5. This is addressed by upgrading to Asciidoctor 1.5.6.1.