Introduce "_formats": "year"

[UlrikeS91]

@olinux, could we create a new "custom" format for year? At the moment, we us a regular expression (([0-9]{4})) for the property "year" in the copyright schema because draft-07 doesn't have this as a regular format (only date and/or time).

This would mean some changes to the generator as well ;)

[UlrikeS91]

This can be postponed to a later point. But good to keep open for now.

[lzehl]

@olinux it would be good to receive your input here.