Feature/add should add contact block

[JettF]

After working with this library, I realized that I needed a way to filter the contacts based on the the values they had for certain CNKeyDescriptor's. For example, say an app needs to only import contacts which have a valid phone number. By setting the shouldAddContact closure to return true when the phone number for a contact is valid, then you can be sure that the contacts the user can pick from will all have valid phone numbers.

@ipraba: if you don't think this is a good idea, I would love to hear why!

Thanks for your time, JF

[AnthonyMDev]

I'm a contributor on this fork as well. I just fixed a bug in reloadContacts() that was causing duplicate contacts to be shown. It seems that the previous implementation of the method was actually just adding the contacts, not reloading them, so if reloadContact() was called twice, all of your contacts would be duplicated. I've set it up now so that it replaces the loaded contacts properly.

We've also changed the filtering closure name from shouldAddContacts to shouldIncludeContacts. I believe this is a clearer description of what this closure does.

[AnthonyMDev]

This PR should expect no more changes. This has been fully tested and is working for us now in production. If you do want to include this feature, you may merge whenever you are ready. We apologize for the multiple commits post-submission.