Add config yaml schema support

[lebrice]

Fixes #288

TODOs:

• [ ] Figure out how to best make this available to the user.

[codecov-commenter]

Codecov Report

Attention: 107 lines in your changes are missing coverage. Please review.

Comparison is base (3194331) 86.44% compared to head (cf5f9a9) 84.55%.

:exclamation: Current head cf5f9a9 differs from pull request most recent head b0c4071. Consider uploading reports for the commit b0c4071 to get more accurate results

Files	Patch %	Lines
...imple_parsing/helpers/serialization/yaml_schema.py	20.74%	107 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #304 +/- ## ========================================== - Coverage 86.44% 84.55% -1.90% ========================================== Files 34 35 +1 Lines 4531 4668 +137 ========================================== + Hits 3917 3947 +30 - Misses 614 721 +107 ```

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