Changed Markdown.hs for Pandoc >=1.10 compatibility

I've tried to run hyakko on my local machine, but it wouldn't install because Pandoc has changed from version 1.9 to 1.10. I've made the required changes to make it run with 1.10. This is my first pull request, and I'm also very new to Haskell, so please excuse any obvious flaws. I'm eager to learn though. Sincerely, Psirus