Add a command dumping missing date of births

src/args.rs

@@ -39,6 +39,9 @@ pub(crate) enum PeopleCommands {
         #[arg(short, long)]
         vcard_file: PathBuf,
     },
+
+    /// Lists the people without date of birth
+    MissingDateOfBirths {},
 }
 
 #[derive(Subcommand)]