Allow 3th party libraries to expand Teaful with extras

[aralroca]

#38

Allow 3th party libraries to expand Teaful with extras. Required for teaful-devtools, etc.

Example devtools

Example of future devtools bridge:

https://codesandbox.io/s/teaful-canary-y72vv?file=/src/store.js

import createStore from 'teaful'

export default function initDevtools() {
  window.__TEAFUL_DEVTOOLS__ = [];

  // Ext = Extra / Extension
  // Function that is executed in each createStore, it allows:
  // - Access to { getStore, useStore, withStore }
  // - Subscriptions: subscribe, notify, unsubscribe.
  // - Return an object (optional) with new store properties.
  createStore.ext(({ getStore }, subscription) => {
    let index = window.__TEAFUL_DEVTOOLS__.push(getStore) - 1;

    // s = subscribe (minified by Teaful)
    // n = notify (minified by Teaful)
    // u = unsubscribe (minified by Teaful)
    subscription.s(".", ({ store, prevStore }) => {
      window.postMessage(
        { source: "teaful-devtools", store, prevStore, index },
        "*"
      );
    });

   // Here is not necessary but is possible to return an object with
   // new things: 
   // return { getExample }
   //
   // These will be available in each createStore with the other:
   // const { getExample, useStore } = createStore()
  });
}

And then to use it:

import createStore from 'teaful'
import initDevtools from 'teaful-devtools' // Not definive dependency

if (process.env.NODE_ENV !== 'production') initDevtools()

const { useStore } = createStore({})
// ...

[aralroca]

Prereleased in 0.9.0-canary.1