Persist dev phone resources

[ayyrickay]

From Phil Nash:
I was surprised to see it tear down all its resources when shutting down. Is there an option to keep them, and to store the config for the dev phone in CLI plugin config, allowing for quick start up the next time? Or, if you are going to tear it down, and the phone number previously had a voice/sms URL configured, put that back?

This seems related to #24, where there are some situations where the developer may want more environmental awareness about the dev phone BETWEEN sessions. These issues should be developed in parallel - if we persist the dev phone resources, that's also a way for us to know what should be torn down (i.e., we could store the dev phone resource URLs and have a boolean on whether a phone is persistent or not. If it's not persistent, we have a direct way to tear them down, including just the friendlyname of the dev phone being available locally.)

I think we SHOULD probably limit each CLI to having one persistent phone at a time, since the point is not really for them to be customisable.