title: Grape: custom validator for each element of an array
body: <p>Is it possible to run a custom validator against each element of array in Grape? I know I can validate whole array with my validator, but I think error messages would be better if I use it for each element.</p>
  
  <p>My parameters look like this:</p>
  
  <pre><code>  "conditions": [
        {
            "field": "interests",
            "operator": "any",
            "value": ['cars', 'cats']
        },
        {
            "field": "age",
            "operator": "gt",
            "value": 25
        }
    ]
  </code></pre>
  
  <p>With <code>requires :conditions, type: Array, valid_conditions: true</code> the validator is run for the whole array. Is it best I can get?</p>