Add --setup (-s) option, like --prepare but runs once per batch

Add a --setup option for the use-case of wanting to do one-off setup
before a set of benchmarks, not the once-per-test setup that --prepare
does. This is useful for the cases noted in the updated documentation.

Per the feedback on https://github.com/sharkdp/hyperfine/pull/448 this
new "--setup" option will steal the "-s" short option from
"--style" (initially this was called "--build" and used the
non-conflicting "-b").

Potential future work:

I'd prefer if this and --cleanup took N number of commands, so you
could do e.g.:

    hyperfine -L rev master,next \
        -b 'git worktree add /tmp/test-{r}'  \
        -b 'make -C /tmp/test-{r} all'  \
	'make -C /tmp/test-{r} test' \
	-c 'git worktree remove /tmp/test-{r}'

I.e. a shortcut around not providing these with &&, which makes things
a bit more readable.

But the --cleanup option doesn't do that, so let's just go with what
it's doing for consistency, so for this you'll now need to do:

    hyperfine -L rev master,next \
        -b 'git worktree add /tmp/test-{r} &&
	    make -C /tmp/test-{r} all'  \
	'make -C /tmp/test-{r} test' \
	-c 'git worktree remove /tmp/test-{r}'

src/benchmark.rs

@@ -185,6 +185,18 @@ fn run_intermediate_command(
     })
 }
 
+/// Run the command specified by `--setup`.
+fn run_setup_command(
+    shell: &Shell,
+    command: &Option<Command<'_>>,
+    show_output: bool,
+) -> io::Result<TimingResult> {
+    let error_output = "The setup command terminated with a non-zero exit code. \
+                        Append ' || true' to the command if you are sure that this can be ignored.";
+
+    run_intermediate_command(shell, command, show_output, error_output)
+}
+
 /// Run the command specified by `--prepare`.
 fn run_preparation_command(
     shell: &Shell,
@@ -263,6 +275,12 @@ pub fn run_benchmark(
         Command::new_parametrized(None, preparation_command, cmd.get_parameters().clone())
     });
 
+    // Run setup command
+    let setup_cmd = options.setup_command.as_ref().map(|setup_command| {
+        Command::new_parametrized(None, setup_command, cmd.get_parameters().clone())
+    });
+    run_setup_command(&options.shell, &setup_cmd, options.show_output)?;
+
     // Warmup phase
     if options.warmup_count > 0 {
         let progress_bar = if options.output_style != OutputStyleOption::Disabled {