title: Checking if a string contains an integer
body: <p>Do you know of a function that can check if a string contains an integer?</p>
  
  <p>Here's how I'd expect it to work:</p>
  
  <pre><code>holds_int("23") // should return true.  
  holds_int("2.3") // should return false.  
  holds_int("qwe") // should return false.
  </code></pre>