Add rough support for .json files

[ejfox]

It might be useful to allow users to try and input .json files. This is seemingly simple but could easily become a rabbit hole because it is a big departure from flat files like csvs.

Dataproofer can then try and transform it into a flat .csv - if this is possible and reasonable, we can just run all our existing tests on it. Some data may be lost in the transformation, make that clear to the user or - even better- guide them through the process of finding where exactly in the .json they would like to flatten things and run their tests.