support append prepend slot (#1709)

packages/input-number/src/input-number.vue

@@ -17,6 +17,12 @@
       :class="{
         'is-active': inputActive
       }">
+        <template slot="prepend" v-if="$slots.prepend">
+          <slot name="prepend"></slot>
+        </template>
+        <template slot="append" v-if="$slots.append">
+          <slot name="append"></slot>
+        </template> 
     </el-input>
     <span
       v-if="controls"