refactored ex NOOP filter & specs into filter base specs

the NOOP filter is now a dynamically defined class into the `spec/filters/base_spec.rb` and the Plugin#lookup method has been adjusted to support dynamically defined plugins.
- `is_a_plugin?` is a private method, will not add tests for it. it gets tested implicitly by correctly testing the `lookup` method. if we ever decide to make it public we'll add tests for it.

- good points for `config_name` and `LogStash::Plugin`