add target option to Affix plugin

docs/_includes/js/affix.html

@@ -62,6 +62,13 @@
          <td>10</td>
          <td>Pixels to offset from screen when calculating position of scroll. If a single number is provided, the offset will be applied in both top and bottom directions. To provide a unique, bottom and top offset just provide an object <code>offset: { top: 10 }</code> or <code>offset: { top: 10, bottom: 5 }</code>. Use a function when you need to dynamically calculate an offset.</td>
        </tr>
+       <tr>
+         <td>target</td>
+         <td>selector | node | jQuery element</td>
+         <td>the <code>window</code> object</td>
+         <td>Specifies the target element of the affix.</td>
+       </tr>
+
       </tbody>
     </table>
   </div><!-- /.table-responsive -->