Escape attributes section with two newlines

Do not interpret attributes section of an extra file if it starts with
two blank lines (previously: one blank line).  This is a breaking
change, however not very drastical.  Seems to be a good compromise
between simplicity and ease of use.

spec/code_objects/extra_file_object_spec.rb

@@ -70,10 +70,10 @@
       expect(file.contents).to eq " #!foobar\nHello"
     end
 
-    it "does not parse out attributes if there are newlines prior to attributes" do
-      file = ExtraFileObject.new('file.txt', "\n# @title\nFOO BAR")
+    it "does not parse out attributes if there are at least two newlines prior to attributes" do
+      file = ExtraFileObject.new('file.txt', "\n\n# @title\nFOO BAR")
       expect(file.attributes).to be_empty
-      expect(file.contents).to eq "\n# @title\nFOO BAR"
+      expect(file.contents).to eq "\n\n# @title\nFOO BAR"
     end
 
     it "sets contents to data after attributes" do