feat(deps): move MathJax configuration to a separate file (#1670)

Move `MathJax` configuration to file `assets/js/data/mathjax.js` will help add extensions.

_includes/js-selector.html

@@ -72,25 +72,7 @@
 
 {% if page.math %}
   <!-- MathJax -->
-  <script>
-    /* see: <https://docs.mathjax.org/en/latest/options/input/tex.html#tex-options> */
-    MathJax = {
-      tex: {
-        /* start/end delimiter pairs for in-line math */
-        inlineMath: [
-          ['$', '$'],
-          ['\\(', '\\)']
-        ],
-        /* start/end delimiter pairs for display math */
-        displayMath: [
-          ['$$', '$$'],
-          ['\\[', '\\]']
-        ],
-        /* equation numbering */
-        tags: 'ams'
-      }
-    };
-  </script>
+  <script src="{{ '/assets/js/data/mathjax.js' | relative_url }}"></script>
   <script src="https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=es6"></script>
   <script id="MathJax-script" async src="{{ site.data.origin[type].mathjax.js | relative_url }}"></script>
 {% endif %}