Roll-up support in IMSC1/imscJS

Good morning/evening,

A pull request [1] against imscJS adds support for roll-up animations.

The proposed implementation is deployed at [2]. Attached is an example document.

This is a client-side implementation, which does not require signaling
in the document. Roll-ups are automatically detected by comparing
lines across ISDs in region elements where tts:displayAlign='after'.

Looking forward to feedback, as always.

Best,

-- Pierre

[1] https://github.com/sandflow/imscJS/pull/67
[2] http://sandflow.com/imsc1proc/index.html