Support optionally-enabled middleware

[dylanmckay]

We could add impl Middleware for Option<M: Middleware>.

Then piplines could be done line so

struct Pipeline {
     encryption: Option<Encryption>,
     compression: Option<Compression>,
}

fn handle(packet: _, pipeline: &mut Pipeline) {
    match packet {
        LoginSuccess => {
            pipeline.encryption = Some(Encryption::new(..));
        },
    }
}

This would be required for protocols such as minecraft, which optionally enables encryption at login.