Generator example group 

New generator example group and recognizing files under spec/lib/generators as generator example group.

Old `require "generator_spec/test_case"` works and is backward compatible. Only when you require only `generator_spec` it will include new example group.

Inspired by

- https://github.com/rspec/rspec-rails/blob/master/lib/rspec/rails/example.rb
- https://github.com/rspec/rspec-rails/blob/master/lib/rspec/rails/example/controller_example_group.rb
- https://github.com/rspec/rspec-rails/blob/master/spec/rspec/rails/example/controller_example_group_spec.rb