Add displaymath to MathJax setup

* lisp/org-html.el (org-export-html-mathjax-template): displaymath
environment and MathJax

Greetings All.

The following patch makes MathJax consider \begin{displaymath} and
\end{displaymath} as math environmetn boundaries. For someone who, like
me, keeps "The not so short introduction to LaTeX2e" alway around, the
displaymath environment is the default way to introduce a block of math.

In fact '\[' and '\]' are also mentioned there but the environment is
used in every single example so the patch minimizes the surprise.

lisp/org-html.el

@@ -290,7 +290,7 @@ You can also customize this for each buffer, using something like
                      \"TeX/noUndefined.js\"],
         tex2jax: {
             inlineMath: [ [\"\\\\(\",\"\\\\)\"] ],
-            displayMath: [ ['$$','$$'], [\"\\\\[\",\"\\\\]\"] ],
+            displayMath: [ ['$$','$$'], [\"\\\\[\",\"\\\\]\"], [\"\\\\begin{displaymath}\",\"\\\\end{displaymath}\"] ],
             skipTags: [\"script\",\"noscript\",\"style\",\"textarea\",\"pre\",\"code\"],
             ignoreClass: \"tex2jax_ignore\",
             processEscapes: false,