title: Display column names that are only factors
body: <p>Is there a way to extract only column names that are factor. For example, in iris dataset, last column is a factor, so only Species (column name and not entire column) should be extracted</p>
  
  <pre><code>&gt; head(iris)
    Sepal.Length Sepal.Width Petal.Length Petal.Width Species
  1          5.1         3.5          1.4         0.2  setosa
  2          4.9         3.0          1.4         0.2  setosa
  3          4.7         3.2          1.3         0.2  setosa
  4          4.6         3.1          1.5         0.2  setosa
  5          5.0         3.6          1.4         0.2  setosa
  6          5.4         3.9          1.7         0.4  setosa
  &gt; str(head(iris))
  'data.frame':   6 obs. of  5 variables:
   $ Sepal.Length: num  5.1 4.9 4.7 4.6 5 5.4
   $ Sepal.Width : num  3.5 3 3.2 3.1 3.6 3.9
   $ Petal.Length: num  1.4 1.4 1.3 1.5 1.4 1.7
   $ Petal.Width : num  0.2 0.2 0.2 0.2 0.2 0.4
   $ Species     : Factor w/ 3 levels "setosa","versicolor",..: 1 1 1 1 1 1
  </code></pre>