Constructor args

[valdrinkoshi]

Addresses #8 by allowing to set one-time properties like container, layout, newChildFn, etc only through the constructor.
Addresses #12 by removing Fn suffix from newChild/updateChild/recycleChild

Initializing VirtualRepeater requires passing at minimum {container, newChild} in the config

const repeater = new VirtualRepeater({
  container: document.body,
  newChild: (item, idx) => {
    const child = document.createElement('div');
    child.textContent = idx;
    return child;
  }
});
repeater.items = new Array(100).fill('item');
repeater.first = 0;
repeater.num = 5;

Initializing VirtualList requires passing at minimum {container, layout, newChild}

const list = new VirtualList({
  layout: new Layout(),
  container: document.body,
  newChild: (item, idx) => {
    const child = document.createElement('div');
    child.textContent = idx;
    return child;
  }
});
list.items = new Array(100).fill('item');

It is still possible to extend both and override any of these configs by overriding either the container, layout setters or the child creation methods, e.g.

class MyList extends VirtualList {
  get _container() { return document.body; }
  set _container(_) {}

  get _layout() { return this.__layout; }
  set _layout(_) { this.__layout = this.__layout || new Layout(); }

  _newChild() { return document.createElement('input') }
}

Custom elements examples have been updated to keep an internal repeater/list instance instead of using the mixins. They forward the setting of public properties to the internal repeater/list e.g. num, first, items.

<virtual-list id="listEl" direction="horizontal"></virtual-list>
<script>
  listEl.items = new Array(10).fill('http://lorempixel.com/200/200/');
  listEl.template = {
    newChild: (item) => {
      const img = document.createElement('img');
      img.width = img.height = 200;
      img.src = item;
      return img;
    }
  };
</script>