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/options.rs

@@ -208,6 +208,9 @@ pub struct Options {
     // Command to use as a reference for relative speed comparison
     pub reference_command: Option<String>,
 
+    // Name of the reference command
+    pub reference_name: Option<String>,
+
     /// Command(s) to run before each timing run
     pub preparation_command: Option<Vec<String>>,
 
@@ -250,6 +253,7 @@ impl Default for Options {
             min_benchmarking_time: 3.0,
             command_failure_action: CmdFailureAction::RaiseError,
             reference_command: None,
+            reference_name: None,
             preparation_command: None,
             conclusion_command: None,
             setup_command: None,
@@ -310,6 +314,9 @@ impl Options {
         options.setup_command = matches.get_one::<String>("setup").map(String::from);
 
         options.reference_command = matches.get_one::<String>("reference").map(String::from);
+        options.reference_name = matches
+            .get_one::<String>("reference-name")
+            .map(String::from);
 
         options.preparation_command = matches
             .get_many::<String>("prepare")