for循环 （@for var from <start> to||through <end>）

• in 关键字 不包过 <end>
• through 关键字 包括 <end>

// for ... from ... to ...
@for $i from 1 to 3{
  .item-#{$i}{
     color:red 
  }
}

// 效果
.item-1{color:red}
.item-2{color:red}

// for ... from ... through
@for $i from 1 through 3{
  .item-#{$i}{
     color:red 
  }
}

// 效果
.item-1{color:red}
.item-2{color:red}
.item-3{color:red}