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/app.rs

@@ -73,6 +73,20 @@ fn build_app() -> App<'static, 'static> {
                 .help("Perform exactly NUM runs for each command. If this option is not specified, \
                        hyperfine automatically determines the number of runs."),
         )
+        .arg(
+            Arg::with_name("setup")
+                .long("setup")
+                .short("s")
+                .takes_value(true)
+                .number_of_values(1)
+                .value_name("CMD")
+                .help(
+                    "Execute CMD before each set of timing runs. This is useful for \
+                     compiling your software with the provided parameters, or to do any \
+                     other work that should happen once before a series of benchmark runs,\
+                     not every time as would happen with the --prepare option."
+                ),
+        )
         .arg(
             Arg::with_name("prepare")
                 .long("prepare")
@@ -155,7 +169,6 @@ fn build_app() -> App<'static, 'static> {
         .arg(
             Arg::with_name("style")
                 .long("style")
-                .short("s")
                 .takes_value(true)
                 .value_name("TYPE")
                 .possible_values(&["auto", "basic", "full", "nocolor", "color", "none"])