The basic model for diagram links. It inherits from joint.dia.Cell with a few additional properties and methods specific to links.

For links there are two crucial attributes: `source` and `target`. It defines the starting point and the end point of the link. It can be defined as follows:

    new joint.dia.Link({
            source: { id: 'source-id' },
            target: { id: 'target-id', port: 'port_id'}
        });

        new joint.dia.Link({
            source: { id: 'source-id' },
            target: { x: 100, y: 100}
        });