v0.6.0

• Improve link layout using stable sort. Thanks, @monfera! (#19)
• Add link.index.
• Add node.index.
• Add node.depth.
• Changed node.x to node.x0.
• Changed node.dx to node.x1 - node.x0.
• Changed node.y to node.y0.
• Changed node.dy to node.y1 - node.y0.
• Changed link.dy to link.width.
• Changed link.sy to link.y0 - link.source.y0 - link.width / 2.
• Changed link.ty to link.y1 - link.target.y0 - link.width / 2.