This solves a few problems:
- We no longer need to write temporary files to disk, as files
passed on the command line with non-Latin text can be read by mplayer.
- We no longer need to deal with mplayer processes left around in
the background that have failed to terminate.
- We don't need to deal with the added complexity that comes with
polling mplayer's status output to determine when the file has finished
playing.
Also add seek_relative(), toggle_pause() and shutdown() as optional
methods on AVPlayer.
- 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.
