Add method for formatting (possibly regional) phone numbers in an "absolute" international way.

Need this for the project I'm working on -- you enter 555-555-5555 and I actually want +15555555555 to send to the backend server so we can send a text message. Similarly if you're on a phone in Spain it should return +34555555555 for "555-555-555".

Let me know what you think ... happy to adjust the naming / whatever.