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>

1 files changed, 6 insertions, 4 deletions

diff --git a/src/ext/voice/threading.rs b/src/ext/voice/threading.rs
index 113c23d..bbbffd1 100644
--- a/src/ext/voice/threading.rs
+++ b/src/ext/voice/threading.rs
@@ -13,7 +13,7 @@ pub fn start(target_id: Target, rx: MpscReceiver<Status>) {
     ThreadBuilder::new()
         .name(name)
         .spawn(move || runner(rx))
-        .expect("Error starting voice");
+        .expect(&format!("[Voice] Error starting target: {:?}", target_id));
 }
 
 fn runner(rx: MpscReceiver<Status>) {
@@ -27,9 +27,11 @@ fn runner(rx: MpscReceiver<Status>) {
             match rx.try_recv() {
                 Ok(Status::Connect(info)) => {
                     connection = match Connection::new(info) {
-                        Ok(connection) => Some(connection),
+                        Ok(connection) => {
+                            Some(connection)
+                        },
                         Err(why) => {
-                            error!("Error connecting via voice: {:?}", why);
+                            warn!("[Voice] Error connecting: {:?}", why);
 
                             None
                         },
@@ -71,7 +73,7 @@ fn runner(rx: MpscReceiver<Status>) {
                 match update {
                     Ok(()) => false,
                     Err(why) => {
-                        error!("Error updating voice connection: {:?}", why);
+                        error!("[Voice] Error updating connection: {:?}", why);
 
                         true
                     },