title: Pass column names as strings to group_by and summarize
body: <p>With dplyr starting version 0.7 the methods ending with underscore such as summarize_ group_by_ are deprecated since we are supposed to use quosures.</p>
  
  <p>See:
  <a href="https://cran.r-project.org/web/packages/dplyr/vignettes/programming.html" rel="noreferrer">https://cran.r-project.org/web/packages/dplyr/vignettes/programming.html</a></p>
  
  <p>I am trying to implement the following example using quo and !! </p>
  
  <p>Working example:</p>
  
  <pre><code>df &lt;- data.frame(x = c("a","a","a","b","b","b"), y=c(1,1,2,2,3,3), z = 1:6)
  
  lFG &lt;- df %&gt;% 
     group_by( x,y) 
  lFG %&gt;% summarize( min(z))
  </code></pre>
  
  <p>However, in the case, I need to implement the columns to group by and summarize are specified as strings. </p>
  
  <pre><code>cols2group &lt;- c("x","y")
  col2summarize &lt;- "z"
  </code></pre>
  
  <p>How can I get the same example as above working?</p>