Refactor speakText generation

[pkra]

From https://github.com/mathjax/MathJax-node/pull/291#issuecomment-279818921

We do add speakText twice if mml with alttext is the input, which technically isn't necessary. [...]

A better solution would be nice, i.e., updating the internal format and leaving it to mathjax. But that would require some refactoring of the chain of events, e.g., splitting the mathjax processing so we can update the internal format; that seems out of scope here.