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.
A bit confused with LogStash::Logger -  not exactly sure how LogStash::Logger should be used in this context... in particular the required target parameter. Currently all logger code is initialized by getting the "current" LogStash channel like https://github.com/elasticsearch/logstash/blob/master/lib/logstash/pipeline.rb#L17

Is the intent to refactor all logger usage with LogStash::Logger? If so maybe we should have a separate PR just for this?