Implementing typed-settings.

[tobiasraabe]

Would close #231.

Changes

This PR attemps to replace pytask's internal settings handling with typed-settings. typed-settings would take-over

• Finding the configuration files.
• Reading, parsing, consolidating the configuration from the cli, the configuration file and potentially environment variables.

Open points

• Is it possible to have an influence on the parameter declarations?
  • Abbreviations for long flags, e.g. -x/--exitfirst from pytest.
  • Possibility of flags compared to switches.
• With attr.ib it is not possible to provide help texts.
  • Implement own type to pass help texts.
• Empty settings classes should not be discarded, but returned as empty.

Todo

• [ ] Display defaults in cli.
• [ ] Check parsing of paths and precedence of cli over config. Check multiline paths in config
• [ ] test markers.