Configure response codes

@eraserhd -- Here's a first pass at the quickest approach to supporting configurable status codes like we discussed.

Assumes that json stub files will contain a comment on the first line noting the response code, followed by an empty line before the response body, eg:

```json
200 OK

{ "hello": "world" }
```

As discussed, this approach could support configurable headers down the road as well.

This'd be a breaking change, as both the expected format of stub JSON files as well as the interface to some of the modules has changed to expect status codes.