Improve endpoint validation

[ghost]

It would be helpful to have valid endpoints store meta data about the endpoint itself, which would allow additional validation rules against the endpoints themselves.

An example rule could be.

[
'achievements' : {
  'requires_term' : True,
  'additional_fields' : ['sortDate']
}
]

This should help with #27 by logging any missing "fields" or "terms" when calling the endpoints.