Add anonymization to field

[Dr. Simon Harrer]

models:
  orders:
    fields:
      customer_email:
        type: string
        format: email
        pii: true
        anonymization: masked # other options: none (default), hashed

I'm not sure whether this should be referenced via the definition.

[jochenchrist]

See #68

I'd go with config parameters at the moment.