Make SvgGraphWriter extendable

[MouthlessGames]

It would be nice if the consumer of the SvgGraphWriter would be able to customize the rendering of shapes. For example, one would be required to set custom attributes that contain the node's id. Making SvgGraphWrtier extendable would help in doing so.

An example of extending the SvgWriter:

class MySvgGraphWriter : SvgGraphWriter {
        protected override void WriteBox(Node node) {
            WriteStartElement("rect");
            WriteFill(node.Attr);
            WriteStroke(node.Attr);
            var curve = node.GeometryNode.BoundaryCurve;
            WriteAttribute("data-id", node.Id); // custom attribute
            WriteAttribute("x", node.BoundingBox.Left);
            WriteAttribute("y", node.BoundingBox.Bottom);
            WriteAttribute("width", curve.BoundingBox.Width);
            WriteAttribute("height", curve.BoundingBox.Height);
            WriteAttribute("rx", node.Attr.XRadius);
            WriteAttribute("ry", node.Attr.YRadius);
            WriteEndElement();
        }
    }

[ghost]

All CLA requirements met.