Add math typesetting support with KaTex (#80)

* add math typesetting support with KaTex
* Simplified the syntax for checking "math" property
* save katex files to local
* use local katex file to render math typesetting
* revert changes in scripts.html and styles.html
* Save math.js as a file, not inline script.
* move it into "assets/js" folder
* Load all necessary scripts and styles in math.html

assets/js/math.js

@@ -0,0 +1,23 @@
+/**
+ * @file
+ * A JavaScript file for render math with KaTex.
+ */
+
+(function () {
+
+    'use strict';
+
+    document.addEventListener('DOMContentLoaded', function () {
+        renderMathInElement(document.body, {
+            // customised options
+            // • auto-render specific keys, e.g.:
+            delimiters: [{left: '$$', right: '$$', display: true}, {
+                left: '$',
+                right: '$',
+                display: false
+            }, {left: '\\(', right: '\\)', display: false}, {left: '\\[', right: '\\]', display: true}],
+            // • rendering keys, e.g.:
+            throwOnError: false
+        });
+    });
+})();