title: Subset XTS by week of the year
body: <p>If I want to subset some month from xts object I can do simply:</p>
  
  <blockquote>
    <p>xtsData["2016-12"]</p>
  </blockquote>
  
  <p>My question is if it is possible to subset a week if I get data in %Y-%W format. This obviously doesn't work:</p>
  
  <blockquote>
    <p>xtsData["2016-49"]</p>
  </blockquote>
  
  <p>I didn't find any info about it. Is there some way to do that or is it impossible?</p>