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.

tests/integration_tests.rs

@@ -499,6 +499,18 @@ fn shows_benchmark_comparison_relative_to_reference() {
         );
 }
 
+#[test]
+fn shows_reference_name() {
+    hyperfine_debug()
+        .arg("--reference=sleep 2.0")
+        .arg("--reference-name=refabc123")
+        .arg("sleep 1.0")
+        .arg("sleep 3.0")
+        .assert()
+        .success()
+        .stdout(predicate::str::contains("Benchmark 1: refabc123"));
+}
+
 #[test]
 fn performs_all_benchmarks_in_parameter_scan() {
     hyperfine_debug()