feat(adoc): create asciidoctor function

plugins/adoc/adoc.plugin.zsh

@@ -0,0 +1,13 @@
+adoc() {
+  if [ -e "docs/index.html" ] && [ "docs/index.html" -nt "README.adoc" ] ; then
+    echo "adoc: 'docs/index.html' is up to date."
+  else
+    asciidoctor README.adoc -o docs/index.html \
+      --require=asciidoctor-diagram \
+      --attribute nofooter \
+      --attribute toc=left \
+      --attribute source-highlighter=highlight.js \
+      --attribute sectnums \
+      --attribute sectnumlevels=2
+  fi
+}