extractors/pipeline make regex-replace capable to insert new-line/carriage-return

## Expected Behavior
When using regex-replace you should be able to create a multiline message by replacing characters with a new line.

Look into this example:

Inserted Content of one field:
```
INFO_1___INFO_2___INFO_3___INFO_4_
```

Should become the following after replacement of `__` to `\n` or similar.
```
INFO_1_
INFO_2_
INFO_3_
INFO_4_
```

The current regex-replace only allow something like, that is not the intention of this replacement.

```
INFO_1_\nINFO_2_\nINFO_3_\nINFO_4_
```

## Context
When receiving data from sources where formatting is limited but the information should be more readable for humans formatting options should work in a very basic way. Creating single fields would only make sense when sub-fields are possible.
This is simple to make messages more readable to humans.

## Your Environment

* Graylog Version: 2.4.6