Add lifecycle event for listen

[aajtodd]

Documentation shows a lifecycle event exists for unlisten on stores. It would be beneficial to also have the listen lifecycle event exposed.

Also some documentation on using lifecycle events in es5 would be helpful.

[goatslacker]

I'd gladly accept a pull request adding a listen lifecycle method.

In ES5 you'd put lifecycle methods like:

function TodoStore() {
  this.on('unlisten', function () {
    // do something
  })
}

// or

var TodoStore = alt.createStore({
  lifecycle: {
    unlisten: function () {
      // do someting
    }
  }
})

[goatslacker]

Oops, reopening for adding a listen lifecycle method.