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/error.rs

@@ -57,7 +57,6 @@ impl Error for ParameterScanError {}
 
 #[derive(Debug)]
 pub enum OptionsError<'a> {
-    RunsBelowTwo,
     EmptyRunsRange,
     TooManyCommandNames(usize),
     UnexpectedCommandNameCount(usize, usize),
@@ -70,7 +69,6 @@ impl<'a> fmt::Display for OptionsError<'a> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match *self {
             OptionsError::EmptyRunsRange => write!(f, "Empty runs range"),
-            OptionsError::RunsBelowTwo => write!(f, "Number of runs below two"),
             OptionsError::TooManyCommandNames(n) => {
                 write!(f, "Too many --command-name options: expected {} at most", n)
             }