Add the concept of "validation_mode"

[joesaunderson]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Documented?	yes
Fixed tickets	N/A
License	MIT

Adds the concept of validation_mode at the default config level.

[Vincz]

Excellent @joesaunderson! You are on the right track! Do you think it would be possible to also be able to manage the validation mode on the object itself (if it's too complicated, we'll try to implement in a future release).
If you can also add the test (with the same object applying both modes and validate the output).