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.

tests/integration_tests.rs

@@ -23,12 +23,12 @@ fn hyperfine_runs_successfully() {
 }
 
 #[test]
-fn at_least_two_runs_are_required() {
+fn one_run_is_supported() {
     hyperfine()
         .arg("--runs=1")
         .arg("echo dummy benchmark")
         .assert()
-        .failure();
+        .success();
 }
 
 struct ExecutionOrderTest {
@@ -109,6 +109,17 @@ impl Default for ExecutionOrderTest {
     }
 }
 
+#[test]
+fn benchmarks_are_executed_sequentially_one() {
+    ExecutionOrderTest::new()
+        .arg("--runs=1")
+        .command("command 1")
+        .command("command 2")
+        .expect_output("command 1")
+        .expect_output("command 2")
+        .run();
+}
+
 #[test]
 fn benchmarks_are_executed_sequentially() {
     ExecutionOrderTest::new()