A thicker line with shadow.

Supported `attrs` properties

Selector | Node | Description
--- | --- | ---
root | SVGGElement | Container of all nodes
line | SVGPathElement | Visible connection of the link
shadow | SVGPathElement | Shadow of the connection

    var shadowLink = new joint.shapes.standard.ShadowLink();
    shadowLink.prop('source', { x: 550, y: 600 });
    shadowLink.prop('target', { x: 500, y: 750 });
    shadowLink.prop('vertices', [{ x: 550, y: 700 }]);
    shadowLink.attr('root/title', 'joint.shapes.standard.ShadowLink');
    shadowLink.attr('line/stroke', '#5654a0');
    shadowLink.addTo(graph);