support post-record processes

There are several issues that could be considered as events that would conditionally happen after a recording is complete:

- https://github.com/amiaopensource/vrecord/issues/592 would require a binary edit to resulting v210/mov files which could be done with mediaconch and sfk.

- https://github.com/amiaopensource/vrecord/issues/482 would need a post-process to analyze if the data received was dv or m2ts and rename the output file accordingly

- a post process could run sccyou and enable subtitles

- and I've heard requests for an mp4 derivative to be made following capture as well.

There's currently a couple after-processes happening now like the qctools report and that graphical image that shows up if you have gnuplot, but let's discuss what after-processes should look like in vrecord.