title: How can I format String alignment-left
body: <p>Firstly I found it is easy for me to format other type, Such as Int, for Example:</p>
  
  <pre><code>let i = 12345
  let s = String(format: "%-10d", 12345)
  //then s = "12345     "
  </code></pre>
  
  <p>The right-hand-side a few space is exactly what I need!</p>
  
  <p>But when I try to implement it to a String (Am I right if I use "%@"?)</p>
  
  <pre><code>let str = "12345"
  let s = String(format: "%-10@", str)
  //then s = "12345" rather than "12345     "
  </code></pre>
  
  <p>How could I do it to achieve the same result?</p>