Add the first draft of an event handler system

examples/dummy.rs

@@ -1,7 +1,14 @@
+use log::debug;
 use env_logger::{Builder, Env};
-use opensmtpd::SmtpIn;
+use opensmtpd::{Entry, EventHandler, MatchEvent, SmtpIn};
+
+fn on_event(entry: &Entry) -> bool {
+    debug!("Event received: {:?}", entry);
+    true
+}
 
 fn main() {
     Builder::from_env(Env::default().default_filter_or("debug")).init();
-    SmtpIn::new().run();
+    let h = vec![EventHandler::new(MatchEvent::All, on_event)];
+    SmtpIn::new().event_handlers(h).run();
 }