Enhancement: Support new-line delimited JSON format/JSON lines

Thank you for the great project. It looks very promising to me.

I currently use `val df = DataFrame.readJsonStr(File("foo.ndjson").readLines().joinToString(",", "[", "]"))`, to read new-line delimited JSON files, which works quite well. However, it would be much more convinient if the API would offer such a function directly. It would be also nice if it would work directly on InputStreams, because `readLines()` is already reading the entire file under the hood.