更简单的外层
又一个客户端js路由器！
用法
var router = new Router ( {
anchorElement : document . getElementById ( 'anchor' ) ,
defaultState : { color : 'green' }
} ) ;
router . route = state => state . color ;
/**
* @return { Promise } promise that accepts with created view.
*/
router . initView = viewName => {
var element = document . createElement ( 'div' ) ;
// ... initialize element
return element ;
