[MAJOR] expose defaults and original Template rendering functions

[tinovyatkin]

In most real life use cases for callbackOnCreateTemplates (like #637), consumer actually want to extend original render a little, without fully overwriting it. However, there is no easy way to do that.
This PR exposes original rendering functions as static readonly property Choices.defaults.templates, so, following code will become possible:

const example = new Choices(element, {
  callbackOnCreateTemplates: () => ({
    input: (...args) =>
      Object.assign(Choices.defaults.templates.input.call(this, ...args), {
        type: "email"
      })
  })
});

MAJOR because it moving undocumented userDefaults into Choices.defaults

[jshjohnson]

👌