[ttml2] multiRowAlign

I plan to adopt a simple solution which is based the semantics of
display:inline-block (see attached). The solution will be to extent
tts:textAlign to accept two <align> value expressions, the first being
 outer alignment, the second being inner alignment.

The implied semantics will be to wrap the content of of a <p/> with a
<span> with implied display of inline-block and the inner alignment.

So, for example, given:

<p tts:textAlign='center left'>One<br/>Two<br/>Three</p>

would effectively map to HTML/CSS as:

<p style="text-align:center"><span
style="display:inline-block;text-align:left">One<br/>Two<br/>Three</span></p>