Registry should expose zeroers, preferred initialization method and type mapping

[fredbi]

go-swagger generator relies on go-openapi/strfmt to generate code.
go-swagger knowledge of strfmt is concentrated here: https://github.com/go-swagger/go-swagger/blob/master/generator/formats.go

It would be a nice refactoring to have this directly provided by the registry.

... first step toward really flexible custom formats.

Similar dependency observed in go-openapi/validate with type reverse lookup (in https://github.com/go-openapi/validate/blob/2a09d726370ca256a065c34ba0def99564d620dc/type.go#L35)