Add support for JSON configuration files

I switched over to crane because of a Fig issue and have invested heavily in JSON configuration files.

I'd like to start using Fig again, but switching from JSON to YAML will be a pain.

It seems pretty trivial for Fig to support JSON as well as YAML for input file format. Thence this Pull Request.
I actually just copied the pattern used by the previous function `test_default_project_name`.

I've refactored `test_json` to follow the pattern set by the other tests.

I'd propose that moving this to `tests/integration/cli_test.py` is a little beyond the scope of this change, but I'm happy to do so, if you would like me to; please confirm.