Add Cache::get_guild_channel

This is an 'extras'-enabled function to get a guild's channel. This will
allow a simple Option pattern match over the result, rather than working
with an Option<Channel<T>>.

Example of old code vs. new code, in the context of an event handler:

Old:

```rs
use serenity::cache::CACHE;
use serenity::model::Channel;

let cache = CACHE.read().unwrap();

let channel = match cache.get_channel(channel_id) {
    Some(Channel::Guild(channel)) => channel,
    Some(Channel::Private(_)) => {
        context.say("Groups and DMs not supported")
            .map_err(|x| println!("Err sending message: {:?}", why);

        return;
    },
    None => {
        context.say("Could not find channel")
            .map_err(|x| println!("Err sending message: {:?}", x);

        return;
    },
}
```

New:

```rs
use serenity::cache::CACHE;

let cache = CACHE.read().unwrap();

let channel = match cache.get_guild_channel(channel_id) {
    Some(channel) => channel,
    None => {
        context.say("Could not find channel")
            .map_err(|x| println!("Err sending message: {:?}", x));

        return;
    },
};
```

2 files changed, 33 insertions, 40 deletions

diff --git a/examples/07_voice.rs b/examples/07_voice.rs
index c352daa..5938c2f 100644
--- a/examples/07_voice.rs
+++ b/examples/07_voice.rs
@@ -5,16 +5,16 @@ extern crate serenity;
 #[cfg(feature = "voice")]
 use serenity::client::{CACHE, Client, Context};
 #[cfg(feature = "voice")]
-use serenity::model::{Channel, ChannelId, Message};
+use serenity::model::{ChannelId, Message};
 #[cfg(feature = "voice")]
 use std::env;
 
-#[cfg(not(feature = "voice"))]
+#[cfg(any(not(feature = "extras"), not(feature = "voice")))]
 fn main() {
-    panic!("Voice not enabled");
+    panic!("'extras' and 'voice' must be enabled");
 }
 
-#[cfg(feature = "voice")]
+#[cfg(all(feature = "extras", feature = "voice"))]
 fn main() {
     // Configure the client with your Discord bot token in the environment.
     let token = env::var("DISCORD_TOKEN")
@@ -36,17 +36,12 @@ fn main() {
     let _ = client.start();
 }
 
-#[cfg(feature = "voice")]
+#[cfg(all(feature = "extras", feature = "voice"))]
 fn deafen(context: Context, message: Message, _args: Vec<String>) {
-    let guild_id = match CACHE.read().unwrap().get_channel(message.channel_id) {
-        Some(Channel::Guild(channel)) => channel.guild_id,
-        Some(_) => {
-            let _ = message.reply("Groups and DMs not supported");
-
-            return;
-        },
+    let guild_id = match CACHE.read().unwrap().get_guild_channel(message.channel_id) {
+        Some(channel) => channel.guild_id,
         None => {
-            let _ = context.say("Can't find guild");
+            let _ = context.say("Groups and DMs not supported");
 
             return;
         },
@@ -72,7 +67,7 @@ fn deafen(context: Context, message: Message, _args: Vec<String>) {
     }
 }
 
-#[cfg(feature = "voice")]
+#[cfg(all(feature = "extras", feature = "voice"))]
 fn join(context: Context, message: Message, args: Vec<String>) {
     let connect_to = match args.get(0) {
         Some(arg) => match arg.parse::<u64>() {
@@ -90,15 +85,10 @@ fn join(context: Context, message: Message, args: Vec<String>) {
         },
     };
 
-    let guild_id = match CACHE.read().unwrap().get_channel(message.channel_id) {
-        Some(Channel::Guild(channel)) => channel.guild_id,
-        Some(_) => {
-            let _ = context.say("Groups and DMs not supported");
-
-            return;
-        },
+    let guild_id = match CACHE.read().unwrap().get_guild_channel(message.channel_id) {
+        Some(channel) => channel.guild_id,
         None => {
-            let _ = context.say("Can't find guild");
+            let _ = context.say("Groups and DMs not supported");
 
             return;
         },
@@ -112,17 +102,12 @@ fn join(context: Context, message: Message, args: Vec<String>) {
     let _ = context.say(&format!("Joined {}", connect_to.mention()));
 }
 
-#[cfg(feature = "voice")]
+#[cfg(all(feature = "extras", feature = "voice"))]
 fn leave(context: Context, message: Message, _args: Vec<String>) {
-    let guild_id = match CACHE.read().unwrap().get_channel(message.channel_id) {
-        Some(Channel::Guild(channel)) => channel.guild_id,
-        Some(_) => {
-            let _ = context.say("Groups and DMs not supported");
-
-            return;
-        },
+    let guild_id = match CACHE.read().unwrap().get_guild_channel(message.channel_id) {
+        Some(channel) => channel.guild_id,
         None => {
-            let _ = context.say("Can't find guild");
+            let _ = context.say("Groups and DMs not supported");
 
             return;
         },
@@ -142,17 +127,12 @@ fn leave(context: Context, message: Message, _args: Vec<String>) {
     }
 }
 
-#[cfg(feature = "voice")]
+#[cfg(all(feature = "extras", feature = "voice"))]
 fn mute(context: Context, message: Message, _args: Vec<String>) {
-    let guild_id = match CACHE.read().unwrap().get_channel(message.channel_id) {
-        Some(Channel::Guild(channel)) => channel.guild_id,
-        Some(_) => {
-            let _ = message.reply("Groups and DMs not supported");
-
-            return;
-        },
+    let guild_id = match CACHE.read().unwrap().get_guild_channel(message.channel_id) {
+        Some(channel) => channel.guild_id,
         None => {
-            let _ = context.say("Can't find guild");
+            let _ = context.say("Groups and DMs not supported");
 
             return;
         },
diff --git a/src/ext/cache/mod.rs b/src/ext/cache/mod.rs
index 7c4d3a5..ac0946b 100644
--- a/src/ext/cache/mod.rs
+++ b/src/ext/cache/mod.rs
@@ -132,6 +132,19 @@ impl Cache {
         self.guilds.get(&id.into())
     }
 
+    #[cfg(feature = "extras")]
+    pub fn get_guild_channel<C: Into<ChannelId>>(&self, id: C) -> Option<&GuildChannel> {
+        let id = id.into();
+
+        for guild in self.guilds.values() {
+            if let Some(channel) = guild.channels.get(&id) {
+                return Some(channel);
+            }
+        }
+
+        None
+    }
+
     pub fn get_group<C: Into<ChannelId>>(&self, id: C) -> Option<&Group> {
         self.groups.get(&id.into())
     }