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%.

❗ 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 ⚠️

Additional details and impacted files

@@            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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