DOM Routines major refactoring, final cut

[dhilt]

This PR provides an additional parameter to the Workflow constructor, which allows to override default behaviour of the DOM Routines.

// routines.ts
import { IRoutines } from 'vscroll';

export class Routines implements Partial<IRoutines> {
  readonly settings: IRoutines['settings'];
  readonly element: HTMLElement;
  readonly viewport: HTMLElement;

  checkElement(element: HTMLElement): void {
    if (!element) {
      throw new Error('HTML element is not defined !!!');
    }
  }
}

// workflow.ts
import { Workflow } from 'vscroll';
import { Routines } from './routines';

...

new Workflow({
      consumer,
      element,
      datasource,
      run,
      Routines
    });

Relates to issue #29.