Cleanvid

Removed a leftover volume filter that should have been removed when I added the afade stuff for 13.

Default pad to "0" and set both fade in and out times to 10ms

Attempt at fixing 13, using afade filter instead of volume on/off filter

Several improvements:

* fix bug where the last subtitle entry would be lost
* ignore -d/--downmix downmix if the audio track(s) are already just stereo
* fix extraction of srt file embedded in video (and allow specifying index e.g., with `-l eng:3` vs just `-l eng`)
* reduce verbosity of output of internally-executed ffmpeg commands

Added `--json` option to write a JSON file with information about input, output and edits.