Handle missing task CLI config

[Wheezily]

Fixes #2.

Summary:

• Create a default task-cli config when ~/.config/task-cli/config.yaml is missing
• Preserve and return existing config contents when a config file already exists
• Add regression tests for missing and existing config files

Validation:

• python -m pytest test_task.py (4 passed)
• python -m compileall task.py commands test_task.py
• Manual CLI check with a temporary HOME: python task.py list creates .config/task-cli/config.yaml and exits normally