Unified configuration loader & Celery settings

- Integrated and unified configuration loader
- YAML configuration format is now supported, and *even recommended!*
- `CELERY_IMPORTS` are no more needed.  It can automatically import all modules under `cliche` package.
- Some docs about Celery

Long story short, you can run Celery worker using the following command:

```console
$ celery worker -A cliche --config dev.cfg.yml  # or dev.cfg.py
```