Implement quizdown into docsify

[andatoshiki]

Hi, really amazing work for this simple quiz form for web. I have tried quizdown for Hugo based documentation sites for educational purposes, but it usually takes a long time on layout modifications, so I migrated to docsify, also an open source projects for direct markdown rendering into html for static documentation sites. I tried pretty much all I can do to fully integrate quizdown into docisfy but never succeeded. Docsify's official has their own tutorial on writing a plugin, which is located here, but I barely understand javascript, can you please try to integrate quizdown feature into docsify? I tried find quiz-related plugins for docsify, but none of them met my demand, yet none of them have such complete quiz features like quizdown. Thank you for this amazing work again.