adding nullify_notifiable as option to dependent_notifications

[christopher-avila]

Issue #, if available:
#140

Summary

1. Adding a new option to available_dependent_notifications_options called nullify_notifiable.
2. Adding a new option to add_destroy_dependency for that key we defined before.
3. Defining into the Notifiable module the new function which will update all the generated_notifications_as_notifiable_for(target_type) to notifiable_type and id to nil.

[simukappu]

Thank you for your contribution!
I think this requested feature is clear, but just wonder what its use cases are. Are there any use cases you would like to nullify notifiable, not deleting notifications?
And, I would appreciate it if you could write RSpec tests of this new :nullify_notifiable option.

Let's discuss in #140. Thank you!