Add before/after framework command hooks

These hooks will each be run prior to or after the command, and will
finish execution before executing the command.

These can be configured in a Framework via:

```rs
client.with_framework(|f| f
    .before(|_context, message, _command_name| {
        println!("Got command '{}'", command_name);
    })
    .after(|_context, _message, command_name| {
        println!("Finished command '{}'", command_name);
    }));
```

This does introduce a backwards compatibility break, by requiring
commands' Context/Message to be borrowed

Upgrade path:

If not using the `command!` macro, modify command signatures from:

```rs
fn some_command(context: Context, message: Message, args: Vec<String>)
```

to

```rs
fn some_command(context: &Context, message: &Message, args: Vec<String>)
```

1 files changed, 1 insertions, 1 deletions

diff --git a/src/ext/framework/command.rs b/src/ext/framework/command.rs
index 0c595b3..e0a4616 100644
--- a/src/ext/framework/command.rs
+++ b/src/ext/framework/command.rs
@@ -4,7 +4,7 @@ use ::client::Context;
 use ::model::Message;
 
 #[doc(hidden)]
-pub type Command = Fn(Context, Message, Vec<String>) + Send + Sync;
+pub type Command = Fn(&Context, &Message, Vec<String>) + Send + Sync;
 #[doc(hidden)]
 pub type InternalCommand = Arc<Command>;