justify-content
- justify-content：center
-
- justify-content：flex-start
-
-
justify-content:flex-end
-
- justify-content：space-around
-
-
justify-content：space-between
-
aligin-items
- aligin-items：flex-start
- align-items：flex-end
- align-items：center
- align-items：stretch(不设置高度时等高显示)
