Implement toCommonHTML for \mathchoice.  Resolves issue #998.
Implement toCommonHTML for \mathchoice.  Resolves issue #998.

GerHobbelt/MathJax,Ivanwangcy/MathJax,anusornc/MathJax,GerHobbelt/MathJax,mcanthony/MathJax,thomasjm/MathJax,mcanthony/MathJax,Edraak/MathJax,jeremykenedy/MathJax,minrk/components-mathjax,dpvc/MathJax,jinntrance/mathjax,jinntrance/mathjax,wikimedia/MathJax,nvp2211/MathJax,minrk/components-mathjax,cben/MathJax,Edraak/MathJax,perseoq/MathJax,nvp2211/MathJax,thomasjm/MathJax,cheerfulstoic/MathJax,anusornc/MathJax,cben/MathJax,jeremykenedy/MathJax,anueshenxing/MathJax,minrk/mathjax-component,thomasjm/MathJax,Ivanwangcy/MathJax,ManrajGrover/MathJax,ManrajGrover/MathJax,perseoq/MathJax,minrk/mathjax-component,thomasjm/MathJax,thomasjm/MathJax,anueshenxing/MathJax,physikerwelt/MathJax,cheerfulstoic/MathJax