Add EBU-TT specifications (#483)
Add EBU-TT specifications (#483)

Adds:
* EBU-TT Part 1 plus aliases to its versions 1.0, 1.1 and 1.2
* EBU-TT Live
* EBU-TT Part 2
* EBU-TT Metadata
* EBU-TT-D v1.0.1 plus aliases from EBU-TT-D to its versions 1 and 1.0.1 (updates URL to https also)

tobie/specref,tobie/specref