Set syntax_hl @requires_stylesheet on root doc

The root document, after nearly all its content is moved to other,
child documents, is unlikely to have anything requiring
highlighting. Thus, the Document object's syntax_highlighter
@requires_stylesheet variable does not get set and a stylesheet
required by any child page(s) will not be written out.

To fix this, while converting the child pages, check whether any of
those pages has a syntax_highlighter with the @requires_stylesheet
variable set. If so, set this variable on the root document. This
causes Asciidoctor to write out the syntax highlighter stylesheet
while it is converting the root document (in
`lib/asciidoctor/convert.rb`).

Closes issue #34.

lib/asciidoctor-multipage.rb

@@ -158,6 +158,11 @@ def convert_document(node)
       # This node is the original full document which has not yet been
       # processed; this is the entry point for the extension.
 
+      # Save a reference to the root document in the converter
+      # instance. This will be used to set the @requires_stylesheet
+      # variable on the root document in the write method.
+      @root_doc = node
+
       # Turn off extensions to avoid running them twice.
       # FIXME: DocinfoProcessor, InlineMacroProcessor, and Postprocessor
       # extensions should be retained. Is this possible with the API?
@@ -594,6 +599,12 @@ def write(output, target)
       ::File.open(outfile, 'w') do |f|
         f.write(doc.convert)
       end
+      if (doc.syntax_highlighter and
+          doc.syntax_highlighter.write_stylesheet? doc)
+        root_doc = doc.mp_root.instance_variable_get(:@root_doc)
+        root_doc.syntax_highlighter.instance_variable_set(
+          :@requires_stylesheet, true)
+      end
     end
   end
 end