Add option to name reference command

Add --reference-name option to give a reference command a name.

This was discussed in the PR that added --reference, #744, but
was unfortunately never implemented.

src/cli.rs

@@ -96,6 +96,14 @@ fn build_command() -> Command {
                     If this is unset, results are compared with the fastest command as reference."
                 )
         )
+        .arg(
+            Arg::new("reference-name")
+                .long("reference-name")
+                .action(ArgAction::Set)
+                .value_name("CMD")
+                .help("Give a meaningful name to the reference command.")
+                .requires("reference")
+        )
         .arg(
             Arg::new("prepare")
                 .long("prepare")