`result.replace("\n", "\r\n")` for `KdlDocument::to_string` on Windows

[dj8yfo]

This might be a problem if files are generated from KdlDocument-s .
If you chose to never ever support Windows, you'd find my deep empathy and understanding.

[zkat]

This should be functionality as part of KdlDocument::fmt &co. Since they're not configurable, I think a good default is "detect existing newlines and use the first one you find. Otherwise, if there's no newlines yet, use the current OS' native newlines".

[zkat]

Update: This is related to #85 and should probably use the same strategy: allowing newline configuration with a FormatConfig object.