Support custom validation and formatting functions

[agarzola]

Property values can be validated when they are set with a custom getter/setter function, but that’s cumbersome. It would be far better to pass a validation object with a validation method for each property that should be validated. This object should be included in the config object, like so:

const Author = new Model({
  name: 'attributes.full_name',
  age: 'attributes.age',
}, {
  validation: {
    name: function (candidate) {
      // function that returns true if `candidate` is valid, and false or an error
      // message if `candidate` is invalid; e.g.
      return (typeof candidate === 'string');
    },
    age: function (candidate) {
      // function that returns true if `candidate` is valid, and false or an error
      // message if `candidate` is invalid; e.g.
      if (typeof candidate !== 'number') {
        return new Error('The age property must be a number.');
      }

      if (candidate > 150) {
        return new Error('Nobody’s that old.');
      }

      return true;
    },
  },

These validation functions should, if present, be invoked before setting a value for the property after which they are named.

Similarly, a formatting object could also be supported for developers to declare methods that accept the value and return a new, formatted value. Useful for ensuring dates are formatted, for example.