wrap mathjax_config into function

... so that one can call it after dynamically adding MathJax
    <script> on page.

Author: etpinard

src/core.js

@@ -18,7 +18,7 @@ require('es6-promise').polyfill();
 require('../build/plotcss');
 
 // inject default MathJax config
-require('./fonts/mathjax_config');
+require('./fonts/mathjax_config')();
 
 // include registry module and expose register method
 var Registry = require('./registry');

src/fonts/mathjax_config.js

@@ -10,26 +10,20 @@
 
 /* global MathJax:false */
 
-/**
- * Check and configure MathJax
- */
-if(typeof MathJax !== 'undefined') {
-    exports.MathJax = true;
-
-    var globalConfig = (window.PlotlyConfig || {}).MathJaxConfig !== 'local';
+module.exports = function() {
+    if(typeof MathJax !== 'undefined') {
+        var globalConfig = (window.PlotlyConfig || {}).MathJaxConfig !== 'local';
 
-    if(globalConfig) {
-        MathJax.Hub.Config({
-            messageStyle: 'none',
-            skipStartupTypeset: true,
-            displayAlign: 'left',
-            tex2jax: {
-                inlineMath: [['$', '$'], ['\\(', '\\)']]
-            }
-        });
-        MathJax.Hub.Configured();
+        if(globalConfig) {
+            MathJax.Hub.Config({
+                messageStyle: 'none',
+                skipStartupTypeset: true,
+                displayAlign: 'left',
+                tex2jax: {
+                    inlineMath: [['$', '$'], ['\\(', '\\)']]
+                }
+            });
+            MathJax.Hub.Configured();
+        }
     }
-
-} else {
-    exports.MathJax = false;
-}
+};