[css-ruby] The default value of ruby-align is undesirable for bopomofo

In the current spec, ruby-align has a default value "space-around". This
value is the usual behavior for Japanese, and is fine for pinyin since it
is just like Latin words. But this value is not fine for bopomofo.

There are justification opportunities between bopomofo characters, just
like other Chinese/Japanese characters. But if we have "ruby-align:
space-around" on them, we will get an undesirable result, because bopomofo
is generally centered when in annotations.

Hence I propose that we add an "auto" value to ruby-align, and make it
default. UA may choose a proper value for it. It may behave as "center" in
Chinese locales, and "space-around" otherwise.

Or alternatively, I guess we can solve it as a rule in the default style
sheet like:
ruby:lang(zh) { ruby-align: center; }

- Xidorn