frint-model: Introduce .get$() for streaming values

[fahad19]

Currently

Right now we can do:

const Todo = createModel({...});

const todo = new Todo({ title: 'My First Todo' });
const title = todo.get('title'); // `My First Todo`

Expected feature

It would be beneficial for form management, if we can observe Model values too:

const title$ = todo.get$('title');

title$.subscribe(x => console.log(x));

todo.set('title', 'Updated title'); // prints out title in console

Benefit

Then for handling forms in component, we could do something like this with frint-react's observe:

import React from 'react';
import { observe, streamProps } from 'frint-react';

const FormComponent = React.createClass({...});

export default observe(function () {
  const todo = new Todo({ title: 'My todo' });

  return streamProps()
    .set(
      todo.get$(),
      todoAttributes => ({ todo: todoAttributes })
    )
    .get$();
})(FormComponent);

[fahad19]

Done in #166