Add an option to enable pretty-printing when writing JSON

[tgregg]

JSON is commonly pretty-printed for human readability. Currently the write command does not provide the option to pretty-print JSON. This can cause confusion when the input file is pretty-printed JSON because the non-pretty-printed output file can be much smaller despite being data model equivalent. It also doesn't provide the most accurate representation of JSON write speed and serialized size in cases where user's JSON data is always pretty-printed.