[improvement] Tag: add type className (#3788)

src/tag/index.less

@@ -13,6 +13,42 @@
     border-radius: @tag-border-radius * 2;
   }
 
+  &--default {
+    background-color: @tag-default-color;
+
+    &.van-tag--plain {
+      color: @tag-default-color;
+    }
+  }
+
+  &--danger {
+    background-color: @tag-dander-color;
+
+    &.van-tag--plain {
+      color: @tag-dander-color;
+    }
+  }
+
+  &--primary {
+    background-color: @tag-primary-color;
+
+    &.van-tag--plain {
+      color: @tag-primary-color;
+    }
+  }
+
+  &--success {
+    background-color: @tag-success-color;
+
+    &.van-tag--plain {
+      color: @tag-success-color;
+    }
+  }
+
+  &--plain {
+    background-color: @tag-plain-background-color;
+  }
+
   &--mark {
     padding-right: .6em;
     border-radius: 0 .8em .8em 0;