added hook for custom validation callback per form field

I needed a hook for a custom validation function.
I could not find this feature in the project, hope I didn't miss it.

Now you can have the property ```validationCallback: "someCallBack(modelValue, form)"``` on your form field.
This callback should return an object with properties `valid (boolean)` and `error (error code)`.
This will only be called, if the tv4 schema validation was successful.