[css-counter-styles] Adding a 'speak-as' descriptor

A while back, Daniel Weck brought up the problem of how to speak out
list counters.  He added a section to CSS3 Speech to deal with this
for the CSS2.1 built-in styles
<http://dev.w3.org/csswg/css3-speech/#lists>, but that obviously
doesn't help when @counter-style opens up the set of styles
infinitely.

To address this, fantasai and I have drafted a 'speak-as' descriptor
for @counter-style, with values that correspond to the three
categories that Daniel split the list styles into, plus a fallback
value. <http://dev.w3.org/csswg/css-counter-styles-3/#counter-style-speak-as>

Thoughts?  Review?  Okay to keep in the draft?

~TJ