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;
    },
};
```

0 files changed, 0 insertions, 0 deletions