allow to specify `scale` in MathJax config

[abate]

I'd like to scale down a little the math fonts. This snippet below should work.

I think this is not possible at the moment without modifying the
config generate by this plugin.

In particular, HTML-CSS should accept scale as possible config parameter

<script type="text/x-mathjax-config"> 
    MathJax.Hub.Config({ 
        "HTML-CSS": { scale: 200, linebreaks: { automatic: true } }, 
        SVG: { linebreaks: { automatic:true } }, 
        displayAlign: "left" });
</script>