Add initial audio support

Audio can be played with support by passing one of the following into
the `Handler::play` method:

`serenity::ext::voice::{ffmpeg, pcm, ytdl}` functions, where
- `ffmpeg` accepts a path (such as a `File`);
- `pcm` accepts a raw reader source;
- `ytdl` accepts a URI, which works with everything youtube-dl supports:
  <https://github.com/rg3/youtube-dl/blob/master/docs/supportedsites.md>

The source can be stopped via [`Handler::stop`].

Receive is supported through [`Handler::listen`], which accepts a
`serenity::ext::voice::AudioReceiver` implementation.

An example is provided in the form of the file located at
`./examples/07_voice.rs`, which can be run by cloning the repo and
performing the command `cargo run --example 07_voice`. Prior to running
the command, set a bot token as the value of the env variable
`DISCORD_TOKEN`. The example supports:

- `deafen`: deafens the bot;
- `join`: joins a voice channel by ID. The example is a primitive
  implementation, and requires the ID of the channel to be passed to the
  bot as a command of `~join 131937933270712320`;
- `leave`: leaves the current voice channel, if in one;
- `mute`: mutes the bot and will continue to play source audio;
- `play`: plays source audio from a URI, through a command like
  `~play https://www.youtube.com/watch?v=5KJjBRm0ElA`;
- `ping`: responds with "Pong!" to ensure the bot is working properly;
- `undeafen`: undeafens the bot, if that's actually a word;
- `unmute`: unmutes the bot.

Documentation for audio can be found at:

<https://serenity.zey.moe/serenity/ext/voice/index.html>

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