[main] sanitize MathJax user configuration

* merge defaults into user configuration
Fixes #309

Author: pkra

lib/main.js

@@ -473,7 +473,9 @@ function ConfigureMathJax() {
   }
   if (MathJaxConfig) {
     adjustArrays(MathJaxConfig);
-    Insert(window.MathJax,MathJaxConfig);
+    // merge the defaults into the user configuration (to sanitize)
+    var currentMathJax = Insert({},window.MathJax);
+    window.MathJax = Insert(Insert({},MathJaxConfig),currentMathJax);
   }
 }