Support "serialize" createField option

[kettanaito]

What

Support serialize property on the field record.

Why

To have a field-specific custom serialization transformer. To deprive the necessity to carry around Form.props.onSerialize for the fields that are meant to have the same serialization transformer.

Specification

1. A field may have a custom onSerialize property upon its creation in the options passed to createField high-order component.
2. There is no serialize option set by default.
3. Custom serialize has the highest priority, and is executed when serializing the fields.
4. Whenever a form-wide Form.props.onSerialize and a field-wide serialize are present, the form's transformer accepts already transformed field serialized value.

How

• Add the serialize property to the createField options
• Grab the serialize from the field record upon dispatching fieldUtils.serializeFields
• Add unit test that custom serialize transformer gets propagated to the field record upon creation
• Add integration test to assert custom serialize transformer is dispatched when a form is serialized
• Add integration test to assert that custom serialize and Form.props.onSerialize complement each other
• Update createField options documentation (+ example of usage)