improving speech-rule-engine integration

This fixes #49 and #50.

* the `result` will now have a separate `speakText` (not sure about the name)
* the MathML will now have the speech string in the alt-text (See inline comments -- I couldn't make the proper way work; for now a regex will have to do.)
Technically, result.speakText should have HTML special characters escaped, but I think they probably won't appear in the speech text, so it isn't strictly necessary.