[ttml2] condition attribute

I've added support for a generic 'condition' attribute [1][2], available
for use with any element type except profile matter vocabulary.

While the primary syntax is now drafted, additional details regarding
semantics of condition expression processing semantics will be needed. But
that can be safely done after FPWD.

[1]
https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml2/spec/ttml2.html#content-attribute-condition
[2] https://dvcs.w3.org/hg/ttml/rev/0933a6108611