Media Overlays playback in non-linear spine items, scope limited to begin / end document boundaries

[danielweck]

Previous / next page commands are disabled when reaching the begin/end boundaries of the ancillary document (non-linear spine item). Similarly, MO playback should be limited to this scope.

See PR (nonLinearSpineItems feature branch):
#211

[danielweck]

Proposal: to test this without pre-recorded audio files, use TTS support in the MO engine.
TODO on this sample: http://readium-diagram.surge.sh/?epub=nonLinear