Add support for Python-based configuration

[dxoigmn]

I am increasingly become disillusioned with YAML configuration (non-standard errors, partial resolving, etc.). It would be really nice to implement Python-based configuration like Detectron2: https://github.com/facebookresearch/detectron2/blob/0ae803b1449cd2d3f8fa1b7c0f59356db10b3083/detectron2/config/lazy.py#L210