title: Taking multiple values in an argument in Clap
body: <p>I'm using <a href="https://docs.rs/clap/2.29.0/clap/" rel="noreferrer">Clap</a> and I'm trying to make it so that a subcommand can take multiple values for the argument. The interface I'm after is:</p>
  
  <pre><code>just use repo [files]
  </code></pre>
  
  <p>An example:</p>
  
  <pre><code>just use radar/dot-files gitaliases ryan-aliases
  </code></pre>
  
  <p>The <code>repo</code> argument here will be the string "radar/dot-files" and the <code>files</code> argument will be a vector of <code>["gitaliases", "ryan-aliases"]</code>.</p>
  
  <p>Here's the code that I'm trying to use:</p>
  
  <pre><code>let matches = App::new("just")
      .version("v1.0-beta")
      .subcommand(
          SubCommand::with_name("use")
              .arg(Arg::with_name("repo").required(true))
              .arg(
                  Arg::with_name("files")
                      .required(true)
                      .multiple(true)
                      .number_of_values(1),
              ),
      )
      .get_matches();
  
  if let Some(matches) = matches.subcommand_matches("use") {
      println!("{:?}", matches.value_of("files").unwrap())
  }
  </code></pre>
  
  <p>This outputs just the first file that I specify, rather than <em>all</em> the files.</p>
  
  <p>How can I make it output all the different files, for an arbitrary number of arguments?</p>