Added support for user defined custom validation callback

[davidstevens37]

This change adds the ability for a user to define a callback to test the property against whatever they want.

the callback is passed 3 arguments: key, value, and the model's scope. This is awesome because you're able to compare against another property (or properties) on the model. return a truthy value back and it will pass the validation or return a falsy value back to fail the validation. the second example works the exact same way except will use the custom message instead of default.

if the object syntax (2) is used, and the user does not define a validate property or the validate property is not a function, the validator will not invoke the callback and the validation will pass.

  validations: {
    lotteryNumber: {
      custom: funciton(key, value, model){
        return model.get('accountBalance') > 1 ? true : false;
      }
    }
  }

  validations: {
    lotteryNumber: {
      custom: {
        validation: funciton(key, value, model){
          return model.get('accountBalance') > 1 ? true : false;
        },
        message: 'You can't win off of good looks and charm.'
      }
    }
  }

[esbanarango]

@davidstevens37 Shouldn't this return the return value from the function?

return typeof customValidator === 'function' ? customValidator() : false;

[davidstevens37]

this is determining whether the customValidation function is located at validations.{property}.custom or validations.{property}.custom.validation (in the case that a custom message is defined), ensuring that the callback is in fact a function and returns the function to be invoked by the _validateCustom method on line 52 where the customValidator will be passed the actual key, value, and scope.

if the customValidator is a function, return it to be executed on line 52. if it is not a function, return false and then line 51 will evaluate to false at which case line 52 will not be executed.

[esbanarango]

Merging! Thank you @davidstevens37! Great work.