Allow to pass placeholder config as HTMLElement

[christophe-g]

Motivation:

• In general: have greater flexibility for manipulating placeholder (e.g. changing its content depending on start-drag scenario).
• In my particular use case: allow polymer data-binding for placeholder as pseudo-code below

<dom-module id="custom-drag-container">
  <template>
      <div id="placeholder">
         <div class="placeholder">[[placeHolderContent]]</div>
      </div>
      <polymer-sortable handle="#drag" placeholder="[[placeholder]]">
          <dom-repeat items="[[arrayItems]]" > 
             <repeated-items></repeated-items>
         </dom-repeat>
      </polymer-sortable>
  </template>
</dom-module>

 class CustomDragElement extends Polymer.Element {
      static get properties() {
        return {
            /*
             * `placeholder` the Element to use as sortable placeholder
             */
             placeholder: {
               type: Object
            },

            /*
             * `placeholderContent` content of placeholder
             */
            placeholderContent: {
              type: String, 
              value: 'new question'
            }

        }

       connectedCallback() {
        super.connectedCallback();
        Polymer.RenderStatus.afterNextRender(this, () => {
          // set placeholder as HTMLElement
          this.placeholder = this.$.placeholder;
          this.addEventListener('sort-start', this._onSortStart);

        });
      }

     _onSortStart(e) {
        console.info(e);
        this.placeholderContent =  ... handle logic to change placeholder content ... 
      }
}

As for #488, will submit a PR soon (with much less code change as this is really a simple change)