[improvement] Popup: add position default value

packages/popup/index.less

@@ -7,20 +7,21 @@
 
   &-popup {
     position: fixed;
-    top: 50%;
-    left: 50%;
     max-height: 100%;
     overflow-y: auto;
     background-color: @white;
     transition: .3s ease-out;
     -webkit-overflow-scrolling: touch;
-    transform: translate3d(-50%, -50%, 0);
+
+    &--center {
+      top: 50%;
+      left: 50%;
+      transform: translate3d(-50%, -50%, 0);
+    }
 
     &--top {
       width: 100%;
       top: 0;
-      right: auto;
-      bottom: auto;
       left: 50%;
       transform: translate3d(-50%, 0, 0);
     }
@@ -28,24 +29,18 @@
     &--right {
       top: 50%;
       right: 0;
-      bottom: auto;
-      left: auto;
       transform: translate3d(0, -50%, 0);
     }
 
     &--bottom {
       width: 100%;
-      top: auto;
       bottom: 0;
-      right: auto;
       left: 50%;
       transform: translate3d(-50%, 0, 0);
     }
 
     &--left {
       top: 50%;
-      right: auto;
-      bottom: auto;
       left: 0;
       transform: translate3d(0, -50%, 0);
     }