Make plain JS configuration a separate section

Fixes #95

configuration.rst

@@ -394,8 +394,14 @@ in this example:
     <script type="text/javascript" src="path-to-MathJax/MathJax.js?config=TeX-AMS_HTML">
     </script>
 
+
+.. _plainjs-config:
+
+Using plain JavaScript
+======================
+
 Starting with MathJax version 2.3, it is possible to set ``window.MathJax`` to
-a configuration object in any Javascript code before MathJax's startup.
+a configuration object in any JavaScript code before MathJax's startup.
 MathJax will then use that object for its initial configuration. For instance
 the previous example becomes:
 
@@ -412,9 +418,9 @@ the previous example becomes:
     <script type="text/javascript" src="path-to-MathJax/MathJax.js?config=TeX-AMS_HTML">
     </script>
 
-Similarly to ``text/x-mathjax-config``, you can enter arbitrary code to execute
-during the configuration phase. You just need to put that code in an
-``AuthorInit`` function:
+Similarly to scripts with the custom type ``text/x-mathjax-config``, you can 
+enter arbitrary code to execute during the configuration phase. You just 
+need to put that code in an ``AuthorInit`` function:
 
 .. code-block:: html