增加折叠效果
Merge branch 'master' of github.com:mindpin/js-workflow-tree
Merge branch 'master' of github.com:mindpin/js-workflow-tree
Merge branch 'master' of github.com:mindpin/js-workflow-tree
Merge branch 'master' of github.com:mindpin/js-workflow-tree
Merge branch 'master' of github.com:mindpin/js-workflow-tree
完善折叠效果