用Emmet写CSS3属性会自动添加前缀

CSS3的很多属性都包含浏览器厂商前缀，用Emmet写CSS3属性会自动添加前缀，比如输入trs 会展开为：

-webkit-transition: prop time;
-moz-transition: prop time;
-ms-transition: prop time;
-o-transition: prop time;
transition: prop time;