Add custom fetcher config option

[jkcorrea]

Adds an option to use your own fetch function:

import { Model, SpraypaintBase } from 'spraypaint'
import ky from 'ky'

const apiClient = ky.extend({})

@Model()
class ApplicationRecord extends SpraypaintBase {
  static fetcher = apiClient
}

[wadetandy]

Mind adding a spec which ensures that setting a single fetcher in the base class will allow both subclasses to use it?