Add a message cache API (#345)

This adds an API for message caching. By default this caches 0 messages per
channel.

This can be customized when instantiating:

```rust
use serenity::cache::{Cache, Settings};

let mut settings = Settings::new();

// Cache 10 messages per channel.
settings.max_messages(10);

let cache = Cache::new_with_settings(settings);
```

After instantiation:

```rust
use serenity::cache::Cache;

let mut cache = Cache::new();
cache.settings_mut().max_messages(10);
```

And during runtime through the global cache:

```rust
use serenity::CACHE;

CACHE.write().settings_mut().max_messages(10);
```

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