Reformat exporter to export to JSON

[alexstandiford]

The exporter was originally created as a way to output registered items and display them directly in a text file, however, as I've worked with Underpin I've come to find that it would be a lot nicer if this was exported as a JSON file, instead. Not only would this be potentially useful in the future for debug purposes, but it may even make it possible to eventually generate entire plugins from a JSON file.