title: How to do colsum and average on the same dataframe
body: <p>I am trying to add a new row to this dataframe below name total. Now for the columns  <code>counts</code>,<code>cost</code>,<code>views</code> are colsums or totals but for <code>average</code>  I want to do average and for average I want to do a custom formula .So how can I do that. I did use the <code>janitor</code> library (<code>adorn_totals("row")</code>)but it just does the sum . Below is the sample dataframe:</p>
  
  <pre><code>data.frame(stringsAsFactors=FALSE,
                  Site = c("Channel1", "Channel2", "Channel3", "Channel4"),
           Counts = c(7637587, 19042385, 72019057, 45742745),
                  Cost = c(199999.993061, 102196.9726, 102574.79, 196174.712132),
      Views = c(3007915, 5897235, 14245859, 24727451),
     Average = c(2.54, 3.23, 5.05543800482653, 2.21111111111111),
                 avg_views = c(7.5197875, 14.7430875, 35.6146475, 48.24)
  )
  </code></pre>