Make certain mandatory rules turn-offable

[handrews]

Some technically correct rules are often violated in practice, and may need to be opt-out to keep the tool useful to a broad audience. For example, GitHub's API description currently fails because their notion of what "format": "uri" allows is not strictly RFC 3986-compliant.

Disabling validation of examples and defaults is already supported with -e, but a more general mechanism is needed. A -i option to relax URI[-reference] syntax to allow IRI[-reference]s is documented but not yet implemented, and would fit in the same category.