Add IneritableAttributes Class and Specs

Fix for issue: https://github.com/wvanbergen/request-log-analyzer/issues/142
Class attributes where not inherited, causing an issue when subclassing file_formats.
This module fixes this. Also added a spec for the new behaviour.
can we just use `extend ClassLevelInheritableAttributes`, instead of doing an `included` hook that in turn `extends ClassMethods`? Not sure what this adds.