code: function checkSync(validations, input, key) {
    var arr = new Checkit(validations).runSync(input);
    if (arr[0] === null) return arr;
    if (arr[0] instanceof CheckitError) {
      return [arr[0].get(key), null]
    }
    return arr;
  }
docs: Synchronously check an individual field against a rule.