Add a type validation on typeset function

Before, the typeset function was called all the time, even when it
was not defined. Thus generating some alerts in the console.
Now, the function is called only when it exists.

See #9

xExtension-LatexSupport/static/mathjax-config.js

@@ -6,5 +6,7 @@ window.MathJax = {
 };
 
 document.body.addEventListener('freshrss:load-more', e => {
-  MathJax.typeset()
+  if ('function' === typeof MathJax.typeset) {
+    MathJax.typeset();
+  }
 });