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 don't know if these new integration tests really provide much extra coverage. Your change is appropriately isolated to just one module. A single integration test of that module (showing that a json file is read and returns the expect dict), should be sufficient.

The test suite is pretty slow as it is, we should avoid making it slower unless the tests actually add new coverage.