Add support for JSON config files

:koala:

Add support for JSON config files

/cc @zendesk/quokka

### Risks
 - None
I would just trust the file extension and do:
![capture](https://cloud.githubusercontent.com/assets/1329716/4148401/9e42abb2-3424-11e4-9934-14f12d7df26e.PNG)
and you can easily handle exceptions, e.g. opt to do it interactively when file type is wrong