Use elements option

```
    <script type="text/x-mathjax-config">
MathJax.Hub.Config({
  elements: ['math'],
  tex2jax: {inlineMath: [['$','$']]}
});
    </script>
```

This allows to convert math expression under specified elements.
We want to specify elements via `Daimon::Markdown::Plugin::Math.script_tag`.
