title: Count the frequency that a value occurs in a dataframe column
body: <p>I have a dataset</p>
  <pre><code>category
  cat a
  cat b
  cat a
  </code></pre>
  <p>I'd like to be able to return something like (showing unique values and frequency)</p>
  <pre><code>category   freq 
  cat a       2
  cat b       1
  </code></pre>