*Backwards compatible* pip package

- Use setuptools to manage package settings
   - Use entry_points for automatic cross-platform executable generation
- Add `rg` module to the system modules list for backwards compatibility
- Expand README.md documentation
*Backwards compatible* pip package

- Use setuptools to manage package settings
   - Use entry_points for automatic cross-platform executable generation
- Add `rg` module to the system modules list for backwards compatibility
- Expand README.md documentation