Write pretty JSON for human eyes?

Toggles skipping lines between attributes of an object and properly indent the written JSON.

Property definitions

json $ JsonSerializer :: pretty_json=
	# Write pretty JSON for human eyes?
	#
	# Toggles skipping lines between attributes of an object and
	# properly indent the written JSON.
	var pretty_json = false is writable
lib/json/serialization_write.nit:53,2--57,36