Add --conclude option with tests

This implementation of --conclude is basically identical
to the existing implementation for --prepare. I have not
adapted any existing tests. All tests which test --conclude
functionality are new.

The --conclude commands will run in the same order in which
they are supplied on the cli in the same manner as --prepare
and there must either be one --conclude command, or else the
same number of --concludes as there are commands.

src/options.rs

@@ -207,6 +207,9 @@ pub struct Options {
     /// Command(s) to run before each timing run
     pub preparation_command: Option<Vec<String>>,
 
+    /// Command(s) to run after each timing run
+    pub conclusion_command: Option<Vec<String>>,
+
     /// Command to run before each *batch* of timing runs, i.e. before each individual benchmark
     pub setup_command: Option<String>,
 
@@ -243,6 +246,7 @@ impl Default for Options {
             min_benchmarking_time: 3.0,
             command_failure_action: CmdFailureAction::RaiseError,
             preparation_command: None,
+            conclusion_command: None,
             setup_command: None,
             cleanup_command: None,
             output_style: OutputStyleOption::Full,
@@ -304,6 +308,10 @@ impl Options {
             .get_many::<String>("prepare")
             .map(|values| values.map(String::from).collect::<Vec<String>>());
 
+        options.conclusion_command = matches
+            .get_many::<String>("conclude")
+            .map(|values| values.map(String::from).collect::<Vec<String>>());
+
         options.cleanup_command = matches.get_one::<String>("cleanup").map(String::from);
 
         options.command_output_policy = if matches.get_flag("show-output") {
@@ -432,6 +440,15 @@ impl Options {
             );
         }
 
+        if let Some(conclusion_command) = &self.conclusion_command {
+            ensure!(
+                conclusion_command.len() <= 1
+                    || commands.num_commands() == conclusion_command.len(),
+                "The '--conclude' option has to be provided just once or N times, where N is the \
+             number of benchmark commands."
+            );
+        }
+
         Ok(())
     }
 }