MathCAT demo for testing chemistry speech

In preparation for Thursday's meeting, I have updated
https://nsoiffer.github.io/MathCATDemo/ to include my beta for speaking
chemistry. Feel free to experiment with this page. I believe this page is
accessible.

This is a beta for chemistry and it hasn't gotten any outside testing, so
please report bugs (there's a bug link at the bottom of the page). MathCAT
goes to great lengths to try and infer Chemistry in the absence of author
intent being specified. This means it may miss seeing something as
chemistry or might infer chemistry when it isn't. I have tuned the internal
threshold for "is this chemistry" so H20 barely triggers it; O2 will not
trigger chemistry speech. Again, if you see something where MathCAT gets it
wrong and you think that's a mistake, please enter a bug report.

For those of you who know TeX's mhchem package, that is probably the
easiest way to enter a formula for testing. E.g., enter "$\ce{H2O}$" for
water. But if you don't care about italics, $H_2O$ will work also.

For chemical elements such as helium, MathCAT is currently emitting "H" and
not "cap h". If you are using NVDA to listen, Settings:Speech:say cap for
capitals will cause it to say "cap h". I believe there is a similar option
for JAWS. In the past some people were insistent that chemistry speech
should always say "cap". If that remains the case, I can add an option to
MathCAT to control that.

I also added a nuclear decay example to the sample speech doc
<https://docs.google.com/document/d/1yjNc1rY3q73Cf1UKPVsnhG0AXjJCu4M8dXAUJdBpRic/edit#>.


I look forward to talking with everyone on Thursday,

    Neil