Add ability to configure asciidoctor extensions to run during page (default),...

Add ability to configure asciidoctor extensions to run during page (default), page-header, and nav processing.
3 jobs for issue-592-extension-modes in 4 minutes and 37 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Init
passed #783936212
yarn

00:01:12

 
  Verify
passed #783936213
lint

00:01:19

passed #783936214
test

00:03:25