Multiline: Support for only merging a specific regexp group

Multiline now supports a "onlygroup" parameter, which replaces the message of subsequent events with only the regexp group specified in the pattern-parameter.

For example:

GARBAGE First line
GARBAGE   Second line

Configuration:
```
filter {
  multiline {
    pattern => "GARBAGE (?<realmessage>.*)",
    what => previous,
    onlygroup => "realmessage"
}
```

Result:

event["message"] => "GARBAGE First line\n  Second line"