v2.1.0 - Plugin-based Architecture

Major Changes:
- Plugin-based architecture with externalized sources.json configuration
- Default CSS selectors with optional per-source override
- GitLab CI/CD for automatic Docker builds
- Simplified configuration (selectors optional for most DDL sites)
- GitHub-compliant neutral tool design

Features:
- Configurable sources via JSON
- Auto-merge with default selectors
- Docker images: registry.gitlab.com/bon-sama/downarr
- Complete documentation and examples