title: mutate_at using function lag but keep first row
body: <p>I am using dplyr in R (with great joy) and want to get the differential of the columns mpg to gear in mtcars. The first row then returns NA (for obvious reason). Instead of this first row being NA I would like it to stay the original value.</p>
  
  <p>I am looking for a clean and efficient way to achieve this (not using join to add the first row to the differntiated values since the code on my own dataset contains many filters and grouped variables). </p>
  
  <p>my code is as follows:</p>
  
  <pre><code>mtcars %&gt;% mutate_at(vars(mpg:gear), funs(. - lag(., 1)))
  </code></pre>
  
  <p>I expect the first row to be mtcars[1] and the rest to be the differential</p>