Add is-relative helper plus docs

Author: lucperkins

docs/documentation/modifiers/helpers.html

@@ -63,5 +63,9 @@
       <td><code>is-sr-only</code></td>
       <td>Hide elements <strong>visually</strong> but keep the element available to be announced by a <strong>screen reader</strong></td>
     </tr>
+    <tr>
+      <td><code>is-relative</code></td>
+      <td>Applies <code>position: relative</code> to the element.</td>
+    </tr>
   </tbody>
 </table>

sass/base/helpers.sass

@@ -274,3 +274,6 @@ $displays: 'block' 'flex' 'inline' 'inline-block' 'inline-flex'
 
 .is-unselectable
   @extend %unselectable
+
+.is-relative
+  position: relative