[improvement] Tag: add color prop (#1946)

packages/tag/index.vue

@@ -7,6 +7,7 @@
     }), {
       'van-hairline--surround': plain
     }]"
+    :style="style"
   >
     <slot />
   </span>
@@ -17,10 +18,23 @@ import create from '../utils/create';
 
 export default create({
   name: 'tag',
+
   props: {
     type: String,
     mark: Boolean,
+    color: String,
     plain: Boolean
+  },
+
+  computed: {
+    style() {
+      if (this.color) {
+        const key = this.plain ? 'color' : 'backgroundColor';
+        return {
+          [key]: this.color
+        };
+      }
+    }
   }
 });
 </script>