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.
This feels more like an integration test to me, since you're having to chdir.

Could I ask you to move this to tests/integration/cli_test.py.

I'd also use `self.command.base_dir` instead of having to chdir, and I'd add another assertion for the contents of the json file. I think that should cover the changes required to support json.