Add support for loading configs through entrypoint names

bob/extension/config.py

bob/extension/data/__init__.py

bob/extension/data/basic_config.py

bob/extension/data/config_with_module.py

bob/extension/data/load-config.py

bob/extension/data/load_config.py

bob/extension/test_config.py

doc/config.rst

doc/links.rst

doc/pure_python.rst

setup.py