Align handling of alttext in mathajx-node and core mathjax

[pkra]

This is a continuation of #207.

We jump through a few hoops to put the (dummy) speech text onto the output. It seems to me this piece could simply move up to core mathjax.

E.g., mathjax-node would simply push the input source to root.alttext and leave the rendering (to aria-label or title tag) to MathJax.

We could also move it entirely upstream, i.e., the idea of "use TeX/ascii as alttext".

[dpvc]

Either of those sound reasonable. Probably providing an option on the input jax to use the original as alttext would be good.

[pkra]

Cleaning up my old issues.