Include mathjax script only on pages that use it

Might improve page load times for pages without math expressions.

Author: ctrueden

_includes/layout/scripts

@@ -6,9 +6,11 @@
 <script src="/assets/js/util.js"></script>
 <script src="/assets/js/jquery.toc.js"></script>
 <script src="/assets/js/lightbox.min.js"></script>
+{% if content contains '$$' -%}
 <script type="text/javascript" async
   src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.7/MathJax.js?config=default">
 </script>
+{% endif -%}
 <!-- Site code -->
 <script src="/assets/js/main.js"></script>
 <script src="/assets/js/dock.js"></script>