Support to load credentials from environment variables

[pv8]

Summary

Add support for loading authentication credentials from environment variables as an alternative to the current config file storage method.

Motivation

Currently, noipy supports storing credentials using the --store option, which saves them to $HOME/.noipy/ directory encoded with Base64. While this works well for many use cases, there are scenarios where environment variables would be more appropriate:

1. Containerized deployments: Docker containers and Kubernetes deployments commonly use environment variables for configuration
2. CI/CD pipelines: Many automation systems prefer environment variables for secrets management
3. Security best practices: Some environments prohibit storing credentials in files on disk
4. Temporary usage: Users who want to run noipy occasionally without persisting credentials to disk
5. Multiple environments: Easier switching between different credentials for dev/staging/production

Implementation Suggestions

The credential loading priority should be:

1. Command line arguments (-u, -p, etc.)
2. Environment variables
3. Stored config file ($HOME/.noipy/)

This ensures backward compatibility while providing the flexibility of environment variables.