def source_path options={}
      @source_path ||= if options[:relative]
        File.join collection_path, "#{self.id}.md"
      else
        File.join root_path, collection_path, "#{self.id}.md"
      end
    end
def source_path options={}
      @source_path ||= if options[:relative]
        File.join collection_path, "#{self.id}.md"
      else
        File.join root_path, collection_path, "#{self.id}.md"
      end
    end
Return absolute path to Markdown file on this machine.