Rework FFmpeg player and add FFmpegOpusAudio
I have extracted some of the base FFmpeg source code into its own
base class and reimplemented the PCM and the new Opus variants.
Support avconv probing
Also fix a few things
Update `__all__`
Fix the bugs
Rework probe functions and add factory function
Probing involves subprocess so it has been reworked into an async
factory function.
Add docs + a few tweaks
* Removed unnecessary read() and is_opus() functions from FFmpegAudio
* Clear self._stdout in cleanup()
* Add 20 second process communication timeout to probe functions
* Capped probe function bitrate values at 512
Change AudioPlayer to use more accurate, monotonic time.perf_counter()
Add lazy opus loading
The library now no longer loads libopus on import, only on
opus.Encoder creation or manually.
Fix review nits
