extensible 0.0.1

Use Extensible on your custom extensions in order to get the following set of advantages overtraditional extensions that override the `Module#extended` method directly: - Calls to `super` are handled internally to ensure all your _extensible_ extensions areproperly initialized. - Your _extensible_ extensions will be automatically able to become the base of other, morespecific extensions while proper initialization is maintained by simply including them. - Bundle several _extensible_ extensions in a single module by including them and they will allbe correctly initialized when extending the bundler module.