Allow plugins to set global defaults

It would be convenient if plugins could define defaults options which could be set globally. The `grok` plugin used to have a `--patterns-path` flag, which has since been removed, but it probably makes more sense to implement this functionality now, given that `--path.settings` already exists.

See some previous discussion with @jordansissel in logstash-plugins/logstash-filter-grok#103.