Use mathjax (#366)

* Use mathjax

* Include MathJax when delimiters are present

---------

Co-authored-by: Stefan van der Walt <stefanv@berkeley.edu>

layouts/partials/head.html

@@ -0,0 +1,14 @@
+{{ if (findRE `(\$.*\$)|(\\\\\[.*\\\\\])|(\\\\\(.*\\\\\))` .Content) }}
+    <script>
+      MathJax = {
+        tex: {
+          displayMath: [['$$', '$$'], ['\\[', '\\]']],
+          inlineMath: [['$', '$'], ['\\(', '\\)']],
+        }
+      };
+    </script>
+    <script type="text/javascript" id="MathJax-script" async
+      src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml.js">
+    </script>
+{{ end }}
+<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Fira+Sans">