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/oggparse.py

@@ -0,0 +1,96 @@
+# -*- coding: utf-8 -*-
+
+"""
+The MIT License (MIT)
+
+Copyright (c) 2015-2019 Rapptz
+
+Permission is hereby granted, free of charge, to any person obtaining a
+copy of this software and associated documentation files (the "Software"),
+to deal in the Software without restriction, including without limitation
+the rights to use, copy, modify, merge, publish, distribute, sublicense,
+and/or sell copies of the Software, and to permit persons to whom the
+Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
+"""
+
+import struct
+
+from .errors import DiscordException
+
+class OggError(DiscordException):
+    """An exception that is thrown for Ogg stream parsing errors."""
+    pass
+
+# https://tools.ietf.org/html/rfc3533
+# https://tools.ietf.org/html/rfc7845
+
+class OggPage:
+    _header = struct.Struct('<xBQIIIB')
+
+    def __init__(self, stream):
+        try:
+            header = stream.read(struct.calcsize(self._header.format))
+
+            self.flag, self.gran_pos, self.serial, \
+            self.pagenum, self.crc, self.segnum = self._header.unpack(header)
+
+            self.segtable = stream.read(self.segnum)
+            bodylen = sum(struct.unpack('B'*self.segnum, self.segtable))
+            self.data = stream.read(bodylen)
+        except Exception:
+            raise OggError('bad data stream') from None
+
+    def iter_packets(self):
+        packetlen = offset = 0
+        partial = True
+
+        for seg in self.segtable:
+            if seg == 255:
+                packetlen += 255
+                partial = True
+            else:
+                packetlen += seg
+                yield self.data[offset:offset+packetlen], True
+                offset += packetlen
+                packetlen = 0
+                partial = False
+
+        if partial:
+            yield self.data[offset:], False
+
+class OggStream:
+    def __init__(self, stream):
+        self.stream = stream
+
+    def _next_page(self):
+        head = self.stream.read(4)
+        if head == b'OggS':
+            return OggPage(self.stream)
+        else:
+            raise OggError('invalid header magic')
+
+    def _iter_pages(self):
+        page = self._next_page()
+        while page:
+            yield page
+            page = self._next_page()
+
+    def iter_packets(self):
+        partial = b''
+        for page in self._iter_pages():
+            for data, complete in page.iter_packets():
+                partial += data
+                if complete:
+                    yield partial
+                    partial = b''