MathJAX v3 #25

With MathJAX v3, the <script type='text/math'> tags are not used anymore. I updated this extension to drop that. While doing it, I realized it can be done much simpler now since the only job of the extension is to prevent the markdown parser from parsing text in math regions. This led to essentially a complete rewrite.

It's in my fork here. Feel free to use it, or parts of it as you need.