title: add_column in tibble with variable column name
body: <p>This code doesn't work to add a column in tibble:</p>
  
  <pre><code>  library(tidyverse)
    df &lt;- data.frame("Oranges" = 5)
    mycols &lt;- c("Apples", "Bananas", "Oranges")
    add_column(df, mycols[[2]] = 7)
  </code></pre>
  
  <p>I get the error message:</p>
  
  <pre><code>  Error: unexpected '=' in "add_column(df, mycols[[2]] ="
  </code></pre>
  
  <p>But this code works:</p>
  
  <pre><code>  add_column(df, "Bananas" = 7)
  </code></pre>
  
  <p>Why?</p>
  
  <p>I don't know the values of 'mycols' ahead of time.  That's why I wrote my code for it to be a variable.  Is this not possible in dplry?</p>