resolves #254 support writing to STDOUT (and other IO objects)

lib/asciidoctor-pdf/converter.rb

@@ -1949,11 +1949,25 @@ def add_outline_level outline, sections, num_levels, page_num_labels, numbering_
   end
 
   def write pdf_doc, target
-    pdf_doc.render_file target
+    if ::IO === target
+      if target == STDOUT
+        # NOTE we must use a substitute for STDOUT as it doesn't support .size()
+        ::StringIO.open do |io|
+          pdf_doc.render io
+          io.rewind
+          target.write io.read
+        end
+      else
+        pdf_doc.render target
+      end
+    else
+      pdf_doc.render_file target
+      # QUESTION restore attributes first?
+      @pdfmarks.generate_file target if @pdfmarks
+    end
     # write scratch document if debug is enabled (or perhaps DEBUG_STEPS env)
     #get_scratch_document.render_file 'scratch.pdf'
-    # QUESTION restore attributes first?
-    @pdfmarks.generate_file target if @pdfmarks
+    nil
   end
 
   def register_fonts font_catalog, scripts = 'latin', fonts_dir