Valdiate arguments config

[samuelcolvin]

Change Summary

Work in progress, currently tests are failing as fields isn't working properly.

Related issue number

This replaces #1378 which I merged prematurely.

Will fix #1376.

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see [changes/README.md](ht tps://github.com/samuelcolvin/pydantic/blob/master/changes/README.md) for details)

[codecov]

Codecov Report

Merging #1663 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1663   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3896      3909   +13     
  Branches       783       788    +5     
=========================================
+ Hits          3896      3909   +13     

Impacted Files	Coverage Δ
pydantic/decorator.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0a465b6...fd8d568. Read the comment docs.