filter to prune fields from events

This is a generic filter which you can use to prune fields from event based on field names and field values.
Regexp in field names and field values are supported.
Dynamic values in field names and field values are supported.
Full coverage rspec tests.

See examples in rspec tests.

Today there are several plugins what I'm aware of that prune fields from events internaly, this plugin can be used to obsolete them.
For example include_metrics & exclude_metrics in output/graphite
or the functionality added in https://github.com/logstash/logstash/pull/411 for filter/kv
The same could be used in filter/csv for example
So instead of putting this pruning logic in specific filters, the prune filter can be used instead.