title: Checking for identical values across multiple columns in a dataframe
body: <p>I have a dataframe with 15 factor columns. I need to create a new column that gives a true/false binary for whether all the values in a given row (for just those columns) are identical. </p>
  
  <p>I've found this strategy...</p>
  
  <pre><code>identical(df[['A']],df[['B']])
  </code></pre>
  
  <p>...but that checks the entire column, whereas I need a solution by row.</p>