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 -->

js/affix.js

@@ -15,7 +15,8 @@
 
   var Affix = function (element, options) {
     this.options = $.extend({}, Affix.DEFAULTS, options)
-    this.$window = $(window)
+
+    this.$target = $(this.options.target)
       .on('scroll.bs.affix.data-api', $.proxy(this.checkPosition, this))
       .on('click.bs.affix.data-api',  $.proxy(this.checkPositionWithEventLoop, this))
 
@@ -32,13 +33,14 @@
   Affix.RESET    = 'affix affix-top affix-bottom'
 
   Affix.DEFAULTS = {
-    offset: 0
+    offset: 0,
+    target: window
   }
 
   Affix.prototype.getPinnedOffset = function () {
     if (this.pinnedOffset) return this.pinnedOffset
     this.$element.removeClass(Affix.RESET).addClass('affix')
-    var scrollTop = this.$window.scrollTop()
+    var scrollTop = this.$target.scrollTop()
     var position  = this.$element.offset()
     return (this.pinnedOffset = position.top - scrollTop)
   }
@@ -51,7 +53,7 @@
     if (!this.$element.is(':visible')) return
 
     var scrollHeight = $(document).height()
-    var scrollTop    = this.$window.scrollTop()
+    var scrollTop    = this.$target.scrollTop()
     var position     = this.$element.offset()
     var offset       = this.options.offset
     var offsetTop    = offset.top