title: Find the most recent value associated with multiple date columns
body: <p>I have a dataframe that looks like this:</p>
  
  <pre><code>    id      date1 value1      date2 value2      date3 value3
  1 1113 2012-01-14     29 2012-09-29     22 2013-10-28     21
  2 1622 2012-12-05     93 2012-12-05     82 2013-01-22     26
  3 1609 2014-08-30     30 2013-04-07     53 2013-03-20    100
  4 1624 2014-01-20     84 2013-03-17     92 2014-01-10     81
  5 1861 2014-10-08     29 2012-08-19     84 2012-09-21     56
  6 1640 2014-03-05     27 2012-02-28      5 2015-01-11     65
  </code></pre>
  
  <p>I want to create a new column that contains whichever value of the three columns "value1", "value2", and "value3" that is the most recent. I don't need to know which date it was associated with.</p>
  
  <pre><code>    id      date1 value1      date2 value2      date3 value3 value_recent
  1 1113 2012-01-14     29 2012-09-29     22 2013-10-28     21           21
  2 1622 2012-12-05     93 2012-12-05     82 2013-01-22     26           26
  3 1609 2014-08-30     30 2013-04-07     53 2013-03-20    100           30
  4 1624 2014-01-20     84 2013-03-17     92 2014-01-10     81           84
  5 1861 2014-10-08     29 2012-08-19     84 2012-09-21     56           29
  6 1640 2014-03-05     27 2012-02-28      5 2015-01-11     65           65
  </code></pre>
  
  <p>Code to create working example:</p>
  
  <pre><code>set.seed(1234)
  id &lt;- sample(1000:2000, 6, replace=TRUE)
  date1 &lt;- sample(seq(as.Date('2012-01-01'), as.Date('2016-01-01'), by="day"), 6)
  value1 &lt;- sample(1:100, 6, replace=TRUE)
  date2 &lt;- sample(seq(as.Date('2012-01-01'), as.Date('2016-01-01'), by="day"), 6)
  value2 &lt;- sample(1:100, 6, replace=TRUE)
  date3 &lt;- sample(seq(as.Date('2012-01-01'), as.Date('2016-01-01'), by="day"), 6)
  value3 &lt;- sample(1:100, 6, replace=TRUE)
  
  df &lt;- data.frame(id, date1, value1, date2, value2, date3, value3)
  </code></pre>