title: Transform year/week to date object
body: <p>String contains 'YEAR WEEK' and I want to transform it with <code>parse_date_time()</code> to a date object but I can't make the code work:</p>
  
  <pre><code>parse_date_time(c("201510"), "YW")
  </code></pre>
  
  <p>I don't have to use <em>lubridate</em>, can be other packages, too.</p>