title: retain dplyr::group_by columns when using tidyr::nest()
body: <p>Take the following example in R</p>
  
  <pre><code>library(dplyr)
  library(tidyr)
  
  mtcars_cyl &lt;- mtcars %&gt;% group_by(cyl) %&gt;% nest()
  </code></pre>
  
  <p>if we look at the column names of <code>mtcars_cyl</code>, we see that <code>cyl</code> is no longer included. </p>
  
  <pre><code>mtcars_cyl$data[[1]] %&gt;% colnames()
  
  [1] "mpg"  "disp" "hp"   "drat" "wt"   "qsec" "vs"   "am"   "gear" "carb"
  </code></pre>
  
  <p>I was expecting to find some method/option for retaining the <code>group_by</code>
  columns within <code>data</code>, but finding a solution is escaping me. I can understand this might be a niche need. As an example, one might want to create a table of each <code>group_by</code> data frame and include <code>cyl</code> as a column in that output.</p>
  
  <pre><code>library(pander)
  mtcars_cyl$data %&gt;% pander::pander()
  </code></pre>
  
  <p>In other cases, when using in combination with <code>purrr</code>, one might need to include the <code>group_by</code> columns in a function call.</p>