[Improvement] Icon: add size prop (#1555)

packages/icon/index.vue

@@ -1,5 +1,5 @@
 <template>
-  <i :class="[b(), `van-icon-${name}`]" :style="{ color }" v-on="$listeners">
+  <i :class="[b(), `van-icon-${name}`]" :style="{ color, size }" v-on="$listeners">
     <slot />
     <div v-if="isDef(info)" :class="b('info')">{{ info }}</div>
   </i>
@@ -14,7 +14,8 @@ export default create({
   props: {
     name: String,
     info: [String, Number],
-    color: String
+    color: String,
+    size: String
   }
 });
 </script>