Implement field names validation in check command

[ernilambar]

Fixes https://github.com/WordPress/plugin-check/issues/360

• get_formatter() takes third optional parameter $valid_fields. Validation will be done only if third argument is passed.
• Implemented only in plugin check command. In other commands WP-CLI will handle itself.

[ernilambar]

cc: @swissspidy @felixarntz @mukeshpanchal27

[swissspidy]

Hmm not really a fan of duplicating logic the formatter already gives us for free and adding more complexity. The current output is maybe not super optimal because we're using it in a loop with some heading before it, but I don't think it's a big deal.

[felixarntz]

@ernilambar +1 to what @swissspidy says. I personally am unsure this is important to validate. If we want to add validation for arguments, there would probably be a more flexible and useful way than only checking for whether any unsupported parameters are passed. I think it works well enough at this point.