Strict model: Set format for date objects

If we have a date field, allow the format of the field to be set using strict model field definitions.

Such as:

[
  'mydate' => [
    'type' => '\Hazaar\Date',
    'format' => 'Y-m-d'
  ]
]

This means that if we convert the date to a string automatically somewhere else, we can set the format here so that no other code needs to be written.