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, 7 insertions, 1 deletions

diff --git a/src/client/dispatch.rs b/src/client/dispatch.rs
index b91b8d7..04bd8c9 100644
--- a/src/client/dispatch.rs
+++ b/src/client/dispatch.rs
@@ -68,7 +68,9 @@ pub(crate) fn dispatch<H: EventHandler + Send + Sync + 'static>(
     shard_id: u64,
 ) {
     match event {
-        DispatchEvent::Model(Event::MessageCreate(event)) => {
+        DispatchEvent::Model(Event::MessageCreate(mut event)) => {
+            update!(event);
+
             let context = context(data, runner_tx, shard_id);
             dispatch_message(
                 context.clone(),
@@ -103,6 +105,8 @@ pub(crate) fn dispatch<H: EventHandler + Send + Sync + 'static>(
 ) {
     match event {
         DispatchEvent::Model(Event::MessageCreate(event)) => {
+            update!(event);
+
             let context = context(data, runner_tx, shard_id);
             dispatch_message(context, event.message, event_handler, threadpool);
         },
@@ -506,6 +510,8 @@ fn handle_event<H: EventHandler + Send + Sync + 'static>(
             });
         },
         DispatchEvent::Model(Event::MessageUpdate(mut event)) => {
+            update!(event);
+
             let context = context(data, runner_tx, shard_id);
             let event_handler = Arc::clone(event_handler);