using font-feature-settings for native MathML on Firefox

Recent discussion on the Mozilla-mathml and latexml list.

Pending some feedback from the MathWG, we should consider using CSS font-feature-settings and follow best practices set by the WG. Browser support is getting quite decent -- http://caniuse.com/#feat=font-feature. IIUC, Firefox 29 (and 31 in particular) should now allow us to use this in the MathML output.

I don't think we even need to re-do the fonts since we're using Plane 0 characters anyway (except for STIX-mathjax and Asana-mathjax; at least STIX might see an update this year).