[tex4ht] MathJax support

[Michal Hoftich]

>
> But this is severe restriction now that users can no longer
> use \Configure{@HEAD} in their .cfg files?
>

You can use \Configure{@HEAD}{some code}, just not
\Configure{@HEAD}{}, because it removes all additions to @HEAD before
that point. This is also why you use all those
\Configure{@HEAD}{\HCode{<meta charset="UTF-8" />\Hnewline}}. It is
not necessary anymore. I think this comes from a old configuration
file for MathJax which predates the HTML5 support in tex4ht.

Best,
Michal