Large numbers get parsed as smaller numbers

This may be a bug/feature of the json library this tool uses, but my CSV had a large number in one of the columns, and it was evidently parsed as different number (also negative). I'm guessing my number was larger than whatever type json2csv's json parse was expecting:

```
$ echo '{"x": 110750513297351875876238378613844499030}' |  /opt/gocode/bin/json2csv -k x
-9223372036854775808
```
