Add support for JSON input/output of Story, Passage, and Story Format objects

[Dan Cox]

As noted in Road to Extwee 2.2, Extwee 2.2 (like Twine-Utils) should be able to input and output JSON representations of stories, passages, and story formats for easier inter-process and library communication.

• Add JSONWriter.js with fromStory(), fromPassage() and fromStoryFormat() + tests.
• Add JSONParser.js with toStory(), toPassage(), and toStoryFormat() + tests.