src/post2html: Ability to reference content of previous fence

I'll be using this to show example HTML code and then output it as actual
HTML to be rendered as part of the article.  Otherwise the HMTL has to be
duplicated and maintained in multiple places.

An alternative is to include a file, but that is much less convenient for
smaller snippets.

src/post2html

@@ -109,6 +109,15 @@ prefmt()
       next
     }
 
+    # reference to the content of the last fence (e.g. if a previous
+    # code block is HTML and we want to render it as an example)
+    /^```/ {
+      gather = !gather
+      if ( gather ) gblock = ""
+    }
+    !/^```/ && gather { gblock = gblock $0 "\n" }
+    !gather && /^ *@LASTFENCE@$/ { print gblock; next }
+
     { print }
   '
 }