title: Mutate_all except some columns
body: <p>I have a dataframe containing a set of variables that I want to lag at different lenghts so that I can use them in regressions later on (instead of lagging one variable at a time manually). </p>
  
  <p>I found this <a href="https://stackoverflow.com/questions/49437319/lag-lead-entire-dataframe-in-r/49437401#49437401">code</a> on Stackoverflow that seems to do the trick:</p>
  
  <pre><code>df = data.frame(a = 1:10, b = 21:30)
  dplyr::mutate_all(df, lag)
      a  b
  1  NA NA
  2   1 21
  3   2 22
  4   3 23
  5   4 24
  6   5 25
  7   6 26
  8   7 27
  9   8 28
  10  9 29
  </code></pre>
  
  <p>The problem is that this lags every column and I have some columns that I don't want to be lagged. How do I adapt the above code so that the columns I don't want to be lagged are excluded? And also how do i lag a different lenghts, now it only lags by 1 as the default setting</p>