Complete skeleton dispatcher

- Complete the dispatcher for the 'introducer' and 'local' command. This
  is currently only a skeleton.
- The 'introducer list' command is added.

src/types/introducer.rs

@@ -56,3 +56,17 @@ impl Introducer {
     }
 }
 
+use std::fmt::{Display, Formatter};
+impl Display for Introducer {
+
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        let uids = self.cert.userids()
+            .map(|u| String::from_utf8_lossy(u.userid().value()).to_string())
+            .collect::<Vec<String>>()
+            .join(", ");
+        write!(f, "Introducer: {}\n", self.cert.fingerprint())?;
+        write!(f, "  UserIDs: {}", uids)?;
+        Ok(())
+    }
+}
+