title: Interleave dataframes after every n column, starting at column x?
body: <p>Say I wanted to extract every 4th element from mtcars, starting from column 2. I could do:</p>
  
  <pre><code>cars_subdf &lt;- mtcars[seq(2, ncol(mtcars), 4)]
  </code></pre>
  
  <p>How could I do the reverse of this? e.g. insert cars_df col 1 as mtcars col 2, then column 2 of cars_df four columns along and so on? I know there's interleave and similar, but they only directly 'zippered' e.g. :</p>
  
  <pre><code>1a 1b 2a 2b
  </code></pre>
  
  <p>Thanks.</p>