title: R - stringr add newline character every two spaced digits
body: <p>Given  </p>
  
  <pre><code>str1 &lt;- "0 1 1 2 2 3 3 4 0 4"
  </code></pre>
  
  <p>I want:</p>
  
  <pre><code>str2 &lt;- "0 1\n1 2\n2 3\n3 4\n0 4"
  </code></pre>
  
  <p>What's the way to do this with stringr?</p>