Add to_string overload for indentation

[tanadeau]

Create an overload of the to_string member function that handles indentation (spaces) and line breaks.

const std::string to_string(int indent) const noexcept

[sillykelvin]

Yeah, I think this is needed when saving a json object to file, it is a great mess if there is no indentation and line breaks.

[nlohmann]

I implemented a pretty-printing serialization. For that, I renamed the function to_string() to dump() and used the syntax of Pythons json.dumps() (see https://docs.python.org/2/library/json.html#json.dump).

This function takes an integer as parameter:

• If the parameter is -1, no pretty printing is done and the result is the same as before.
• If the parameter is 0, then newlines are added after elements in arrays and objects.
• If the parameter is positive, then this number of spaces is used as indentation.