Improved support as replacement for npm-based json2csv

[ahochsteger]

I'm looking for a go-based replacement of the npm-based json2csv but there are some features missing that still require some pre-processing of data before it can be fed into this go-based implementation:

• Support arrays of objects as input, not only line-delimited JSON
• Make it possible to output all keys found in the data without having to specify each key
• Specify a quote value (see "-q") to quote fields with

There are more functionality that may be added but these are the most important ones that prevent me from switching. It would be great to add support for them (and maybe others) so a switch of the implementations is easier to do.