Add logstash-util-zeromq to logstash-plugins

The file `lib/logstash/util/zeromq.rb`, used and present in https://github.com/logstash-plugins/logstash-output-zeromq, https://github.com/logstash-plugins/logstash-input-zeromq and https://github.com/logstash-plugins/logstash-filter-zeromq should be moved to an external dependency to prevent unintended side effects like the one described in https://github.com/logstash-plugins/logstash-input-zeromq/pull/18. It would also prevent the file `lib/logstash/util/zeromq.rb` from being developed in different directions and thereby to become incompatible.

This would help to resolve https://github.com/logstash-plugins/logstash-input-zeromq/issues/17 and https://github.com/logstash-plugins/logstash-output-zeromq/pull/13.

You find my proposed version of logstash-util-zeromq in https://github.com/breml/logstash-util-zeromq. It is based on the latest version of `lib/logstash/util/zeromq.rb` (from https://github.com/logstash-plugins/logstash-input-zeromq/commit/4876e7cd7115028c60f36aa1bdc838eddcf682fe) and the logstash plugins skeleton (license et al should be correct), it includes new rspec tests, not currently present in the above mentioned zeromq plugins and finally, yes I have signed the CLA.

After this "plugin" is moved to logstash-plugins, all the three mentioned logstash plugins should be updated to include this new dependecy insted of the file `lib/logstash/util/zeromq.rb`.

@avishai-ish-shalom, @purbon and @suyograo FYI