Adds ability to use custom fields in the profile module.

This change uses the `set_api_fields` method to add form fields that were posted in the form using the `Shop_CheckoutData::get_custom_fields()` format to the customer model before saving.

You will be able to use fields by the name `custom_fields['x_column_name']` in your form along side the `shipping[]` or `billing[]` ones and they will save to the customer model as you might expect.