title: In purr 'group_map', how to keep group information in the result
body: <p>In 'group_map', how to add group information to the result list ?
  As attached image, the result list include three tibble ,how to add group information (6/4/8) to seperate tibble ?  (<code>mtcars$cyl %&gt;% unique()</code> &gt; '6 4 8')</p>
  <pre><code>library(tidyverse)
  mtcars %&gt;% 
    group_by(cyl) %&gt;% 
    group_map(~ head(.,2)) 
  </code></pre>
  <p><a href="https://i.stack.imgur.com/dkTiy.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/dkTiy.png" alt="enter image description here" /></a></p>