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);
```

1 files changed, 9 insertions, 0 deletions

diff --git a/src/lib.rs b/src/lib.rs
index 196a3e0..74f050a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -202,6 +202,15 @@ lazy_static! {
     /// println!("{}", CACHE.read().user.id);
     /// ```
     ///
+    /// Update the cache's settings to enable caching of channels' messages:
+    ///
+    /// ```rust
+    /// use serenity::CACHE;
+    ///
+    /// // Cache up to the 10 most recent messages per channel.
+    /// CACHE.write().settings_mut().max_messages(10);
+    /// ```
+    ///
     /// [`CurrentUser`]: model/struct.CurrentUser.html
     /// [`Cache`]: cache/struct.Cache.html
     /// [cache module documentation]: cache/index.html