Mark --outfile parameter as required

src/bin/keygen.rs

@@ -21,6 +21,7 @@ fn main() -> Result<(), Box<error::Error>> {
                 .long("outfile")
                 .value_name("PATH")
                 .takes_value(true)
+                .required(true)
                 .help("path to generated file"),
         ).get_matches();