refactor: Adjust to latest commit of simple plugin

Now reiterates significantly less code from the simple plugin. Also, adjusted
  the default for extract_standard_markdown when copy_standard_markdown is true.

tests/sites.bash

@@ -3,7 +3,7 @@ startdir=$PWD
 for file in tests/fixtures/*
 do
     cd "$startdir/$file"
-    mkdocs build
+    mkdocs -v build
     # unfortunately MkDocs writes the run date in the last few lines of index:
     head -n -3 site/index.html > site/croppedindex.html
     rm site/index.html