Collect unknown keywords as annotations

[handrews]

And try to avoid memory exhaustion attacks, which were possible
even with out this change but potentially more likely to happen
by accident with it (e.g. unrecognized applicator with a very
large subschema).

Fixes #698.

[karenetheridge]

Thank you, this resolves an inconsistency in the 2019-09 specification document which indicates that the 'format' vocabulary should default to disabled, yet we should still collect the keyword value as an annotation (which requires implementations to special-case 'format' evaluation as "always evaluate this keyword, even if the vocabulary is false, but now the vocabulary boolean specifies only whether the validation check should be performed").

[handrews]

@karenetheridge yeah, if format is "unknown" (and "vaguely known but not entirely supported" can be lumped in with "unknown", collecting it now matches the default behavior. There's still the awkward "best effort" validation support option but we're stuck with that for now as it's what is realistically implemented today and there aren't yet viable alternatives.