Add FFmpegOpusAudio and other voice improvements

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

discord/voice_client.py

@@ -68,11 +68,10 @@ class VoiceClient:
 
     Warning
     --------
-    In order to play audio, you must have loaded the opus library
-    through :func:`opus.load_opus`.
-
-    If you don't do this then the library will not be able to
-    transmit audio.
+    In order to use PCM based AudioSources, you must have the opus library
+    installed on your system and loaded through :func:`opus.load_opus`.
+    Otherwise, your AudioSources must be opus encoded (e.g. using :class:`FFmpegOpusAudio`)
+    or the library will not be able to transmit audio.
 
     Attributes
     -----------
@@ -111,7 +110,7 @@ class VoiceClient:
         self.timestamp = 0
         self._runner = None
         self._player = None
-        self.encoder = opus.Encoder()
+        self.encoder = None
 
     warn_nacl = not has_nacl
     supported_modes = (
@@ -356,7 +355,9 @@ class VoiceClient:
         ClientException
             Already playing audio or not connected.
         TypeError
-            source is not a :class:`AudioSource` or after is not a callable.
+            Source is not a :class:`AudioSource` or after is not a callable.
+        OpusNotLoaded
+            Source is not opus encoded and opus is not loaded.
         """
 
         if not self.is_connected():
@@ -368,6 +369,9 @@ class VoiceClient:
         if not isinstance(source, AudioSource):
             raise TypeError('source must an AudioSource not {0.__class__.__name__}'.format(source))
 
+        if not self.encoder and not source.is_opus():
+            self.encoder = opus.Encoder()
+
         self._player = AudioPlayer(source, self, after=after)
         self._player.start()
 
@@ -444,4 +448,4 @@ class VoiceClient:
         except BlockingIOError:
             log.warning('A packet has been dropped (seq: %s, timestamp: %s)', self.sequence, self.timestamp)
 
-        self.checked_add('timestamp', self.encoder.SAMPLES_PER_FRAME, 4294967295)
+        self.checked_add('timestamp', opus.Encoder.SAMPLES_PER_FRAME, 4294967295)