title: Make a group_indices based on several columns
body: <p>I would like to generate indices to group observations based on two columns. But I want groups to be made of observation that share, at least one observation in commons. I can see how to make groups based on observations that share both observation in common, but not just one of them.</p>
  
  <p>For example, with the data frame :</p>
  
  <pre><code>dt &lt;- data.frame(id=1:10,
               G1 = c("A","A","B","B","C","C","C","D","E","F"),
               G2 = c("Z","X","X","Y","W","V","U","s","T","T"))
  </code></pre>
  
  <p>I would like to get a column </p>
  
  <pre><code>1,1,1,1,2,2,2,3,4,4
  </code></pre>
  
  <p>I tried with group_indices from dplyr, but haven't managed it.</p>