Add delegate option so tap can be handled anywhere

Developers should not have to modify the package in order to handle the user tapping the alert.

 - Does not break existing code of developers using this package
 - Improves usability and portability by allowing developers to use the Pod
 - Uses the delegate/protocol pattern the same way many built-in Cocoa classes do
 - Allows developers to implement the handler differently in different parts of the app or for different alerts