Allow to pass function to message option

[fnky]

The message option is limited to strings, which makes it non-trivial to deal with arrays, objects, etc.

var acceptedValues = [
  { id: 1, name: 'Banana' },
  { id: 2, name: 'Apple' },
  { id: 2, name: 'Mango' }
]

// Fruit is chosen through select which uses ids for value attribute
var constraints = {
  fruit: {
    within: acceptedValues.map(function (o) { return o.id }),
    message: function (id) {
      return '^No such fruit ' + getFruitById(id) + ' available.'
    }
  }
}

[ansman]

Interesting, this definitely seems to be in the spirit of validate.js!