title: cleanly paste columns that contain NAs
body: <p>I am trying to paste 2 text columns together. The problem is that some of the values in each column are NA and if that is the case, I don't want the NA as part of my pasted string. Here is an example of what I mean</p>
  
  <p>some data:</p>
  
  <pre><code>dat &lt;- data.frame("col1" = c("stuff", "stuff", "stuff", NA, NA),
             "col2" = c("things", NA, "things", "things", NA))
  
  dat
  
     col1   col2
  1 stuff things
  2 stuff   &lt;NA&gt;
  3 stuff things
  4  &lt;NA&gt; things
  5  &lt;NA&gt;   &lt;NA&gt;
  </code></pre>
  
  <p>This is what I need:</p>
  
  <pre><code>col1   col2          col3
  1 stuff things stuff; things
  2 stuff   &lt;NA&gt;         stuff
  3 stuff things stuff; things
  4  &lt;NA&gt; things        things
  5  &lt;NA&gt;  &lt;NA&gt;          &lt;NA&gt;
  </code></pre>
  
  <p>I can use paste() and then clean up the mess with gsub() but I was looking for a better one-liner.</p>
  
  <p>Thanks</p>