Add support for one run with --runs=1

Extend the --runs=N option added in d78c33b (Added options to specify
the max/exact numbers of runs., 2018-09-09) to support --runs=1
instead of dying with a usage error.

This is useful to do ad-hoc testing of your commands while they might
still have syntax errors, or just for doing one run where you don't
care about the stddev.

Before:

    $ /usr/bin/hyperfine -s basic -r 1 -L n 5,10 'sleep 0.{n}'
    Error: Number of runs below two

After (-s basic) also works:

    $ hyperfine -r 1 -L n 5,10 'sleep 0.{n}'
    Benchmark 1: sleep 0.5
      Time (abs ≡):        500.6 ms               [User: 0.6 ms, System: 0.0 ms]

    Benchmark 2: sleep 0.10
      Time (abs ≡):        100.8 ms               [User: 0.7 ms, System: 0.0 ms]

    Summary
      'sleep 0.10' ran
        4.97 ± 0.00 times faster than 'sleep 0.5'

This likewise combines correctly with -m and -M, probably not very
useful, but if you're tweaking an existing command-line:

    $ hyperfine -m 1 -M 1 -L n 5,10 'sleep 0.{n}'
    Benchmark 1: sleep 0.5
      Time (abs ≡):        500.6 ms               [User: 0.5 ms, System: 0.0 ms]

    Benchmark 2: sleep 0.10
      Time (abs ≡):        100.6 ms               [User: 0.6 ms, System: 0.0 ms]

    Summary
      'sleep 0.10' ran
        4.98 ± 0.00 times faster than 'sleep 0.5'

The "± 0.00" output in "faster than" should probably be adjusted too,
or we could keep it for consistency. I didn't implement that because
this is the first time I do anything in Rust, and I ran out of
template to copy when wanting to quickly implement this in
write_benchmark_comparison() in main.rs.

src/benchmark.rs

@@ -391,7 +391,10 @@ pub fn run_benchmark(
     // Compute statistical quantities
     let t_num = times_real.len();
     let t_mean = mean(&times_real);
-    let t_stddev = standard_deviation(&times_real, Some(t_mean));
+    let mut t_stddev = 0.0;
+    if times_real.len() != 1 {
+        t_stddev = standard_deviation(&times_real, Some(t_mean));
+    }
     let t_median = median(&times_real);
     let t_min = min(&times_real);
     let t_max = max(&times_real);
@@ -409,7 +412,16 @@ pub fn run_benchmark(
     let user_str = format_duration(user_mean, Some(time_unit));
     let system_str = format_duration(system_mean, Some(time_unit));
 
-    if options.output_style != OutputStyleOption::Disabled {
+    if options.output_style != OutputStyleOption::Disabled && times_real.len() == 1 {
+        println!(
+            "  Time ({} ≡):        {:>8}  {:>8}     [User: {}, System: {}]",
+            "abs".green().bold(),
+            mean_str.green().bold(),
+            "        ".to_string(), // alignment
+            user_str.blue(),
+            system_str.blue()
+        );
+    } else if options.output_style != OutputStyleOption::Disabled {
         println!(
             "  Time ({} ± {}):     {:>8} ± {:>8}    [User: {}, System: {}]",
             "mean".green().bold(),