Provide filter to sub field names

Just a feature request. It would be nice if we could do something like:

```
filter {
rename_fields {
root => "[headers]" // In case we don't want to touch all fields
# replace all forward slashes with underscore
"replace" => [ "/", "_" ]
# regex replace
"regex" => { "unwanted_prefix_(.*)" => "$1" }
# deprecate the dedot filter
"dedot" => true
}
}
```

There's a ruby alternative laid out in [this stackoverflow question](http://stackoverflow.com/questions/31068169/how-to-remove-underscores-in-field-names-with-logstash/31120218#31120218).