取消键总是dismiss，只能通过Obj-C Category来将行为改变为popViewController

[boyuanx]

虽然现在这样改变取消按钮的行为可行，但是为了Quality of Life，不知道作者方不方便把popViewController加进去。或者让用户通过cancelButton.action可以直接自定义cancelButton行为。

[ko1o]

@boyuanx 通过成为searchViewController的delegate 实现以下代理方法即可取代dismiss.

/**
 Called when cancel item did press, default execute `[self dismissViewControllerAnimated:YES completion:nil]`.

 @param searchViewController search view controller
 */
- (void)didClickCancel:(PYSearchViewController *)searchViewController;