title: data.table version of tidyr::unite
body: <p>I need to unite several columns with delimiter in my huge <code>data.table</code>. So I use <code>unite</code> from <code>tidyr</code> package for it.</p>
  
  <p>Do you know if there is any <code>data.table</code> optimized version for that?</p>
  
  <pre><code>library(data.table)
  data &lt;- data.table(id=1:10, col1=11:20, col2=21:30, col3=31:40)
  print(data)
  
  library(tidyr)
  data &lt;- unite(data, "col_test", col1, col2, col3)
  print(data)
  </code></pre>