feature/json-control-indentation-spaces

[njlr]

Allow user to control indentation when serializing JSON.

let text = """{"items":[{"id":"Open"},null,{"id":25}],"x":{"y":2},"z":[]}"""
let json = JsonValue.Parse text
json.ToString(indentationSpaces = 3)

An optional argument is used to keep things backward compatible.