Refactor CLI to use flags instead of positional arguments

- Change all commands to use flags (--bucket, --prefix, etc.)
- Add --config flag to backup command
- Support VAULTIK_CONFIG environment variable for config path
- Use /etc/vaultik/config.yml as default config location
- Add test/config.yaml for testing
- Update tests to use environment variable for config path
- Add .gitignore for build artifacts and local configs
- Update documentation to reflect new CLI syntax

test/config.yaml

@@ -0,0 +1,27 @@
+age_recipient: age1xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+source_dirs:
+  - /tmp/vaultik-test-source
+  - /var/test/data
+exclude:
+  - '*.log'
+  - '*.tmp'
+  - '.git'
+  - 'node_modules'
+s3:
+  endpoint: https://s3.example.com
+  bucket: vaultik-test-bucket
+  prefix: test-host/
+  access_key_id: test-access-key
+  secret_access_key: test-secret-key
+  region: us-east-1
+  use_ssl: true
+  part_size: 5242880  # 5MB
+backup_interval: 1h
+full_scan_interval: 24h
+min_time_between_run: 15m
+index_path: /tmp/vaultik-test.sqlite
+chunk_size: 10485760  # 10MB
+blob_size_limit: 10737418240  # 10GB
+index_prefix: index/
+compression_level: 3
+hostname: test-host