title: Count unique occurrences within data frame
body: <p>Let the table be as follows:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>v1</th>
  <th>v2</th>
  <th>v3</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>A</td>
  <td>B</td>
  <td>A</td>
  </tr>
  <tr>
  <td>B</td>
  <td>B</td>
  <td>A</td>
  </tr>
  <tr>
  <td></td>
  <td>A</td>
  <td>C</td>
  </tr>
  <tr>
  <td>D</td>
  <td>C</td>
  <td>D</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>What i want R to create a table for number of occurences of unique values for each column:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>v1</th>
  <th>v2</th>
  <th>v3</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>A</td>
  <td>1</td>
  <td>1</td>
  </tr>
  <tr>
  <td>B</td>
  <td>1</td>
  <td>2</td>
  </tr>
  <tr>
  <td>C</td>
  <td>0</td>
  <td>1</td>
  </tr>
  <tr>
  <td>D</td>
  <td>1</td>
  <td>0</td>
  </tr>
  </tbody>
  </table>
  </div>