Overridable pod's init function

[jonyrock]

I propose to make it possible for chartwer pod creators:

import { ChartwerkPod } from '@chartwerk/base';

export class MyChartwerkPod extends ChartwerkPod<any, any> {
  constructor(el: HTMLElement, _series: any[] = [], _options: any = {}) {
    super(d3, el, _series, _options);
    this.init();
  }

  // overriding of super.init();
  protected init() {
    super.init();
    // my init logic here
  }

This approach is very common to UI libs. Overriding of init function is crucial for polymorphic usage of pods. For example, we want to send our pod to a class which initializes pods in a spacial order, but client code can't reach that logic.
It other hand, we are not obligated to overload init method, which is also good.