Add option for parsing documents in arrays

[mrodrig]

In order to add support for mrodrig/json-2-csv#102, we should add some sort of option that can be specified in order to determine whether documents appearing in arrays should also be parsed for keys.

If documents in arrays are parsed, then json-2-csv will have a harder time converting the CSV back to the original JSON, which is why an option would be nice.

When documents in arrays are not parsed, json-2-csv is currently using JSON.stringify to get the value, which means that the exact JSON for arrays can be recreated when converting back to JSON from CSV.

Providing an option allows the user to select the behavior that fits their use case.