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;
    },
};
```

1 files changed, 13 insertions, 0 deletions

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())
     }