Add Evented trigger didScroll

[snewcomer]

An end user would do something like this in your codebase....

import RouterScrollService from 'ember-router-scroll/services/router-scroll';

export default class RouterScroll extends RouterScrollService {
  init() {
    super.init(...arguments);

    this.on('didScroll', () => ...)
  }
}

close #245

[Ravenstine]

Sweet as!