Whitelist managed via YAML. 403 code returned.

A yaml file is parsed for environment-specific list of IPs to whitelist. Default is config/whitelist.yml but the actual relative path to the file can be passed in the rack config line.

A non-existent environment in the YAML is treated as an "allow all". I.e. if you don't include 'development' in the YAML, then all IPs are allowed for development.