节点跟随
节点（A）可以跟随另一个节点（B）移动而移动，这种情况下，B节点称为宿主节点，A节点称为跟随节点
- host - 宿主节点
- addFollower - 添加跟随节点
- clearFollowers - 清除跟随节点
示例
设置节点A跟随Host，节点B跟随A
var host = graph.createNode("Host");
var a = graph.createNode("A follow Host", -50, 50);
a.host = host;
var b = graph.createNode("B follow A", 50, 50);
b.host = a;
运行效果：
