Poptip support global setting

examples/routers/poptip.vue

@@ -17,7 +17,7 @@
                     </Poptip>
                 </div> -->
                 <div>
-                    <Poptip title="提示标题" content="标签一的内容" transfer>
+                    <Poptip title="提示标题" content="标签一的内容">
                         <Button id="aaa">点击显示</Button>
                         <div slot="content">
                             <Button @click='loadData(15)'>15条数据</Button>

src/components/poptip/poptip.vue

@@ -96,7 +96,9 @@
             },
             transfer: {
                 type: Boolean,
-                default: false
+                default () {
+                    return this.$IVIEW.transfer === '' ? false : this.$IVIEW.transfer;
+                }
             },
             popperClass: {
                 type: String