- the new API operates on AVTags so it can support TTS as well as
audio files
- added a simple "run for each file" implementation for mpv and mplayer.
- will need to test handling of unicode filenames on Windows
- the old mpv and mplayer code is currently not active
This allows us to add a docstring to .append() so users can see
the names of the arguments that are being passed, and means we
don't have to remember to prepend run_ when calling a hook.
