Better import handling/detection

The `createWriteStream` now has an additional `jsonpath` option, which translates to the first argument of [JSONStream.parse()](https://www.npmjs.org/package/JSONStream). This allows selections like `rows.*` for json object formatted data. 

The input detection of `createWriteStream` tries to detect json and the json style (e.g ndjson, object, array). So it basically tries to detect the new `jsonpath` option.

It also detects different separators of csv files (`\t,;|`). So it won't fail on them anymore.

The dat import command now recognizes `.csv` and  `.json` file endings.

I haven't included tests yet.