A double line link.

Supported `attrs` properties

Selector | Node | Description
--- | --- | ---
root | SVGGElement | Container of all nodes
line | SVGPathElement | Inner connection
outline | SVGPathElement | Outer connection

    var doubleLink = new joint.shapes.standard.DoubleLink();
    doubleLink.prop('source', { x: 500, y: 600 });
    doubleLink.prop('target', { x: 450, y: 750 });
    doubleLink.prop('vertices', [{ x: 500, y: 700 }]);
    doubleLink.attr('root/title', 'joint.shapes.standard.DoubleLink');
    doubleLink.attr('line/stroke', '#30d0c6');
    doubleLink.addTo(graph);