cea608tojson: expose "unbuffered" property
In this mode, cues are output as soon as they are ready for display, without a duration. This can be useful in live mode, when downstream is OK with determining the duration after the fact, through clear=True.
The consequence of this is that the current roll-up window will be output repetitively, it is up to downstream to deal with that how it prefers.
Edited by Mathieu Duponchelle