* Make prototype with enterpolation before parsing
* Add test for option.from_yaml() with missing env not required
* Make some cosmetic changes to _resolve_config_env_markers()
* Add test for option.from_ini() missing envs not required
* Skip schema test cause it requires internet connection
* Add tests for .from_yaml() for config and config option
* Add tests for .from_ini() for config and config option
* Add example for os.environ.setdefault() and envs interpolation
* Add/update docs on environment variables interpolation
* Update changelog
* Add implementation and basic test
* Add full test coverage + bugfix
* Add test coverage for .from_yaml() method
* Update setup.py, tox and dev requirements
* Stop running pydantic tests on Python 3.5 and below
* Remove pydantic from tox Python < 3.6
* Add example and docs
* Update features block
* Add extra test
* Update changelog
