phonenumbers should be optional dependecy

[ar7n]

It will allow to choose between phonenumbers and phonenumberslite, how it is done in phonenumber_field.

Optional twilio and nexmo dependencies also make sense.

[CuriousLearner]

What are the advantages of supporting phonenumberslite?

phonenumbers is there because we wanted to validate phone_numbers and not to have those validations handled in this library.

[ar7n]

size of phonenumberslite package is 5Mb vs 32.8M of phonenumbers. Also phonenumbers has larger memory footprint. Validation works same in both packages.

[vaughnkoch]

Thanks for this useful package! I agree - dependencies like twilio and nexmo should be optional. This is especially important for docker images, e.g. in my image:

148K nexmo
28M twilio
48M phonenumbers

[CuriousLearner]

Thanks for the input. I agree.

There are two things here, The first is making the backend dependencies optional depending on which SMS backend developer opts for. I will cover this in #87

The other one is to replace phonenumbers with phonenumberslite which I'll handle in another PR, unless of course anyone wants to put in a PR for this :)

Thanks again! :D