Allow user to specify fields to overwrite for KV

[suyograo]

The KV plugin will happily overwrite pre-existing fields in the event stream when splitting, including fields that have special meaning such as "type." Proposed feature set would include three options for kv:

1. Allow all overwrites (current behavior)
2. Disallow generated fields to overwrite existing fields.
3. Allow user to specify an array of fields which kv generator can overwrite, still allowing new fields to be introduced.

From https://logstash.jira.com/browse/LOGSTASH-2236

[fbaligand]

It would also be nice to have a fourth option : "add parsed values" to the field, making the target field as an array.