Add option for/Upgrade to MathJax 3.0

[ritobanrc]

mdBook currently uses MathJax 2 by default, I think it would make sense to have an option in the default index.hbs file to use MathJax 3.0. However, MathJax 3.0 is incompatible with 2.0 (see for http://docs.mathjax.org/en/latest/upgrading/v2.html more details). Maybe it makes sense to leave the current mathjax_support option as it is, and add a mathjax_v3_support, i.e.

{{#if mathjax_support}}
   <!-- MathJax -- what we have now -->
   <script async type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>
{{else if mathjax_v3_support}}
    <!-- Adding version 3 -->
    <script type="text/javascript" id="MathJax-script" async  src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml.js">
{{/if}