Fluentd plugin (#210)

* add beginnings of fluentd plugin

* working fluentd output plugin for loki

* refactor fluentd plugin

- update the plugin to support extra_labels, label_keys, line_format
  and drop_single_key options
- update rspec tests
- update readme
- build custom docker image for docker-compose
- default to systemd input by default.

* update readme todo section

* remove extra license file