Support custom validators in the constraints schema

[ansman]

This would be useful for when you want a validator to be used in just a special case.

The format could be:

{
  "<attribute>": {
    "<validator name>": function(value, options, key, attributes) {
      ...
    }
  }
}

[jeron-diovis]

+1 for this enhancement, would be very useful.

[kyeotic]

Is this still planned?

[ansman]

@tyrsius Yes, an implementation is being discussed and implemented in #135

[omansak]

Status ?