Add mathjax support in jekyll

I finally make it works.

I think I should read the mathjax documents carefully.

   <!-- MathJax -->
   {% if site.enable_mathjax %}
   <script type="text/javascript" src="//cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML">
   </script>
   {% endif %}

 <script type="text/x-mathjax-config">
MathJax.Hub.Config({
  tex2jax: {inlineMath: [['$','$'], ['\\(','\\)']]}
});
</script>
<script type="text/javascript" async src="path-to-mathjax/MathJax.js?config=TeX-AMS_CHTML"></script>

src should be // instead of http:// or https://

Mathjax start

explain something related to https://.