Add ability to set all fieldnames to lowercase

MIGRATED FROM: https://logstash.jira.com/browse/LOGSTASH-732

Text:

```
Since there's no way to make searching on field names case insensitive, a filter/feature to set all fieldnames to lowercase would be helpful. For example, if you have two different log sources and one uses "hostname" as a field name and the other uses "Hostname" as a field. By setting all fieldnames to lower case, it would allow joining of log source searches.
```