[ttml2] expressing feature/extension restriction/extension

Updated TTML2 ED [1] with change set [2] (note I've had some problems with
accessing this link, i think a temporary problem with mercurial server) in
order to:

   - add @extends and @restricts to ttp:{feature,extension}
   - add ttp:permitFeature{Narrowing,Widening} parameter attributes

[1] https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml2/spec/ttml2.html
[2] https://dvcs.w3.org/hg/ttml/rev/9a3008473dc8