I've forked the loader - and added some more stuff:

1. isSensor - to create sensor fixtures
2. userdata - to set an "id" value which can be used to identify fixtures or bodies
3. group index (which does not seem to work with love2d)

Feel free to pull the changes back in.

One question: Why do you create a .json file? This requires an additional loader for the JSON data. What about simply creating a .lua file as shapes. It would even be possible to remove the loader code completely.
Or is there some advantage I don't see?