ARIB-TT

I've recently been apprised of the work by ARIB on extensions to TTML via
ARIB-TT, pages number 57 and following in [1].

We should try to learn more about who is doing this work and interface with
them regarding a number of extensions they have defined (in an ARIB defined
namespace).

[1] http://www.arib.or.jp/english/html/overview/doc/2-STD-B62v1_0-1p2.pdf