[css-counter-styles] OM model for @counter-style rules

The section of css-counter-styles now includes snippets of an OM interface [1]:

> The CSSCounterStyleRule interface represents a ‘@counter-style’ rule.
> 
> interface CSSCounterStyleRule : CSSRule {
>   readonly attribute CSSStyleDeclaration style;
> }
> style of type CSSStyleDeclaration, readonly
>
> The style attribute must return a CSSStyleDeclaration block that
> contains the descriptors specified within the @counter-style rule.

I think if we're just looking for a generic getter/setter interface
for descriptors on @-rules we should define that.  This is duping the
unfortunate interface from CSSFontFaceRule and I don't think that's a
good pattern to repeat.  The list of descriptors on a @counter-style
rule really has nothing to do with style rules.

Cheers,

John

[1] http://dev.w3.org/csswg/css-counter-styles/#the-csscounterstylerule-interface