title: tidyr use separate_rows over multiple columns
body: <p>I have a data.frame where some cells contain strings of comma separate values: </p>
  
  <pre><code>d &lt;- data.frame(a=c(1:3), 
         b=c("name1, name2, name3", "name4", "name5, name6"),
         c=c("name7","name8, name9", "name10" ))
  </code></pre>
  
  <p>I want to separate those strings where each name is split into its own cell. This is easy with </p>
  
  <pre><code>tidyr::separate_rows(d, b, sep=",") 
  </code></pre>
  
  <p>if it is done for one column a time. But I can't do this for both columns "b" and "c" at the same time, since it requires that the number of names in each string is the same. Instead of writing </p>
  
  <pre><code>tidyr::separate_rows(d, b, sep=",") 
  tidyr::separate_rows(d, c, sep=",") 
  </code></pre>
  
  <p>Is there a way to do this in a one-liner, for e.g. with apply? Something like</p>
  
  <pre><code>apply(d, 2, separate_rows(...)) 
  </code></pre>
  
  <p>Not sure how to pass the arguments to the <code>separate_rows()</code> function. </p>