Custom Functions

[yedidmh]

Solution for #43.

Adds support for Function.custom() which accepts a user-defined function and passes it the field value, expecting an object to be returned with the fields valid and msg.

var validate = schema({
  'birthday': Function.custom(x => ( { valid: moment(x).isValid(), msg: 'Invalid datetime' } ))
});

Not a pure predicate because I want to allow defining an error message.

[tizmagik]

This looks useful. Any chance to get this in @molnarg ?