feat(config): add config file loading (#13)

Introduces a config module that reads TOML from --config, the
OPENSHELL_IMAGE_BUILDER_CONFIG env var, or the platform XDG
config directory (in that order). Missing or empty files fall
back to built-in defaults; an explicit path that does not exist
fails fast. Adds -v/-vv verbosity flags controlling info/debug
log output. Documents config schema and file discovery in README.

Closes #2

Signed-off-by: Philippe Martin <phmartin@redhat.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: feloy