- use provided language and voices when playing on Mac
- fix hang in waiting for termination
- allow players to return a rank for a given tag,
which will allow for the best matching player to be chosen
depending on the context (eg, prioritize one player for videos,
one tts player for certain voices, etc)
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.
