Prism: Supports candidate_keys for each occurrence

[davidwessman]

• When translating with some rails methods or relative keys, it
  allows for multiple possible keys at runtime.
• A relative key like .success inside a controller action can resolve
  to either <controller_name>.<action_name>.success or
  <controller_name>.success.
• By allowing the scanner to return these candidate keys as well
  and check for an occurrence in missing_keys we can support them
  both.