Validate configs

[MichaelChirico]

          I would actually err on the side of safety and suggest consciously using `rm()` or `local()` if temporary variables are needed.

Re validation:

linters should be a list of linters (we already have that validation somewhere).
Exclusions should also be checked "manually".
The rest seems okay to check against types of default_settings.

But most importantly, no unused variable should be defined. And yes, the validation should be executed after loading the config from either source.

Originally posted by @AshesITR in #2177 (comment)

There's also more discussion in the PR about validation.