This function adds MathJax to the page and typeset the math expressions (if
found) in the content `...`

. It only needs to be called once in an app
unless the content is rendered *after* the page is loaded, e.g. via
`renderUI()`

, in which case we have to call it explicitly every
time we write math expressions to the output.

`withMathJax(...)`

- ...
any HTML elements to apply MathJax to

```
withMathJax(helpText("Some math here $$\\alpha+\\beta$$"))
#> <span class="help-block">Some math here $$\alpha+\beta$$</span>
#> <script>if (window.MathJax) MathJax.Hub.Queue(["Typeset", MathJax.Hub]);</script>
# now we can just write "static" content without withMathJax()
div("more math here $$\\sqrt{2}$$")
#> <div>more math here $$\sqrt{2}$$</div>
```