Use a builder instead of a raw function

This pattern will, in the future, allow the registration of events
handlers and context objects.

examples/dummy.rs

@@ -1,6 +1,7 @@
 use env_logger::{Builder, Env};
+use opensmtpd::SmtpIn;
 
 fn main() {
     Builder::from_env(Env::default().default_filter_or("debug")).init();
-    opensmtpd::run();
+    SmtpIn::new().run();
 }