[Idea] Validate config and --strict mode

[ericcornelissen]

This issue is just to share an idea for a feature for the WebMangler CI: add a subcommand to validate a configuration file (find missing fields that are required; find present fields that are unknown; find fields with invalid values; etc.) and/or add a --strict flag that only runs WebMangler if the configuration file is completely valid.

[ericcornelissen]

In #271 an new field will be added to the WebMangler CLI configuration, named reporters, that should(?) be given one (1) or more reporters. It may make sense to validate that there is at least on reporter provided (if configured).