title: R: Round down dates to first day of the week
body: <p>I have a dataframe where one of the columns contains dates (some dates appear multiple times). I want to aggregate the dates by week. The best way I can think of this is to round down the dates to the nearest Monday. How can I round down dates? How can I transform this list of dates into weeks?</p>
  
  <pre><code>2016-04-04
  2016-04-05
  2016-04-06
  2016-04-07
  2016-04-08
  2016-04-09
  2016-04-10
  2016-04-11
  2016-04-12
  2016-04-13
  2016-04-14
  </code></pre>
  
  <p>Expected output should be this:</p>
  
  <pre><code>2016-04-04
  2016-04-04
  2016-04-04
  2016-04-04
  2016-04-04
  2016-04-04
  2016-04-04
  2016-04-11
  2016-04-11
  2016-04-11
  2016-04-11
  </code></pre>