[TTML2] Issue-228: Ruby text support

I've added the following review note to Issue-228:


Alignment with the proposed approach for Rubys in HTML5 has been achieved semantically but not syntactically with the current draft proposal. One of the impacts of this is that, in order to support Rubys, processors must also support #nested-span. Another is that an extra level of translation is needed to create an HTML5 equivalent ISD including Rubys.


Nigel