closedcaption: implement cea608tojson element

This element outputs the same format expected by tttocea608 in json mode.

It notably differs from cea608tott in that it only uses libcaption's low-level API, as it needs to maintain its own view of the current state of the screen, and make fine-grained decisions as to when to output data and how to timestamp it.

It covers a large portion of the 608 spec, with the exception of a few features that probably haven't ever seen widespread usage, those are listed in a TODO list at the top.

It has been tested with a reference file produced by CEA and covers all the features it demonstrates.