Dynamic include_keys and exclude_keys for kv filter

Implementation for issue https://github.com/elasticsearch/logstash/issues/1617.

When specifying `include_keys` and/or `exclude_keys` for use in the kv filter, a user can include dynamic key names. For example, if an event has a `key` field and the user wants to parse out a value using the kv filter, the user should be able to say `include_keys: [ "%{key}" ]`. The same goes for `exclude_keys`.