title: "tidyr like" fill na from different column
body: <p>I've a data frame with missing values is some column (who doesn't).
  For example:</p>
  
  <pre><code>df &lt;- data.frame(x = c(2,NA,4), y = 5:7)
  df
     x y
  1  2 5
  2 NA 6
  3  4 7
  </code></pre>
  
  <p>I would like to replace the missing value with a value of a different column. Obviously there are a lot of ways to do so, for example:</p>
  
  <pre><code> df %&gt;%
     mutate(x = ifelse(is.na(x), y, x))
  
    x y
  1 2 5
  2 6 6
  3 4 7
  </code></pre>
  
  <p>However, I am looking for something more elegant, like </p>
  
  <pre><code>df %&gt;% fill(x,y) 
  </code></pre>
  
  <p>but couldn't find anything. Does something like this exist? </p>
  
  <p>Thanks!</p>