message: Ignore the --show-output flag used by libtest
diff: @@ -831,6 +831,10 @@ impl<M: Measurement> Criterion<M> {
  .long("nocapture")
  .hidden(true)
  .help("Ignored, but added for compatibility with libtest."))
  + .arg(Arg::with_name("show-output")
  + .long("show-output")
  + .hidden(true)
  + .help("Ignored, but added for compatibility with libtest."))
  .arg(Arg::with_name("version")
  .hidden(true)
  .short("V")
  