Make it possible to import Account SID and Auth Token from env variables

[philnash]

If a developer is already storing their Account SID and Auth Token in env variables then we should be able to use them.

Implementation:

If Account SID/Auth Token is detected in the environment, prompt to ask whether to use them.
If declined, move on to prompt for the credentials.

Include a flag, --import-credentials, which skips the prompt if the credentials are present (for automation).