Plugin context data

[olanod]

Related to #8 and possibly #13, there should be an easy way to have some state that is shared across requests(e.g. a DB connection). One way is attaching data to context that is passed to requests, middlewares and possible plugin life-cycle hooks.

#[hook]
async fn on_created(cx: &mut Context) {...}

#[vlugin]
async fn handler(req: Request, cx: &Context) -> Response {...}