add support for ttml profile registry

[palemieux]

Specifying ttp:profile="http://www.w3.org/ns/ttml/profile/imsc1/text" on tt yields the following error message when using the ttml2 or ttml1 models:

Invalid {http://www.w3.org/ns/ttml#parameter}profile value 'http://www.w3.org/ns/ttml/profile/imsc1/text'.

[skynavga]

That's reasonable behavior, since these models don't know anything about IMSC. However, it may be better to emit a warning than a validation error. What we need is support for a new imsc11 model.

[palemieux]

However, it may be better to emit a warning than a validation error.

+1

[skynavga]

Support for an imsc11 model was added in #229 et al. Renaming this issue to focus on support for TT Profile Registry introduced in w3c/ttml2#1123.

[skynavga]

See https://www.w3.org/TR/ttml-profile-registry/.