README suggests that `working.directory` can be omitted from
`config.json`, but doing this will cause a KeyNotFoundException.
The dictionary lookup has been modified to include a default value of
`'.'` which is according to README.
In addition to loading the config via a `config.json` file, this change allows it to be loaded via an environment variable called `CZDS_CONFIG`. This makes it straightforward to deploy to services like Heroku, which tend to use env vars for config. See https://12factor.net/ for details.
