Move 'ignorable' flag to Mapping base class, make it a ClassVar

[soininen]

PR #570 introduces ignorable flag to ImportMapping. It works the same way as the same flag in ExportMapping: if the imported value is None, we still process child mappings if ignorable == True. Unlike the flag in ExportMapping, the flag is a constant ClassVar. ExportMapping allows ignorable to be set but the functionality is not exposed to users in Exporter as there is really no need to modify it.

Since both import and export mappings have the same flag and it can be made immutable, we should move the flag to the Mapping base class and implement it there as ClassVar like in ImportMapping.