Allow to target a collection element in ValidateAttribute

[thibouf]

Here is a sample usage :

  [Validate("List should not be empty", nameof(CheckList), applyToCollection: true)]
  public List<int> intList;

  bool CheckList()
  {
      return intList == null || intList.Count < 3;
  }

  [Validate("List element should not be zero", nameof(CheckList2Index), applyToCollection: false)]
  public List<int> intList2;

  bool CheckList2Index(int index)
  {
      return intList2[index] == 0;
  }

Which would display :

Linked to issue #68