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>)
```

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