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/main.rs

@@ -193,6 +193,8 @@ fn build_hyperfine_options<'a>(
         (None, None) => {}
     };
 
+    options.setup_command = matches.value_of("setup").map(String::from);
+
     options.preparation_command = matches
         .values_of("prepare")
         .map(|values| values.map(String::from).collect::<Vec<String>>());