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.
Fair enough, I would still add a couple assertions though (either here, or in other tests)

```python
self.assertEqual(command.get_config_path(), 'fig.json')
expected = {...}
self.assertEqual(command.get_config(command.get_config_path(), expected)
```