[new feature] Loading: add default slot

packages/picker/index.js

@@ -2,6 +2,7 @@ import { use } from '../utils';
 import { prevent } from '../utils/event';
 import { deepClone } from '../utils/deep-clone';
 import { pickerProps } from './shared';
+import { BLUE } from '../utils/color';
 import Loading from '../loading';
 import PickerColumn from './PickerColumn';
 
@@ -170,13 +171,7 @@ export default sfc({
     return (
       <div class={bem()}>
         {Toolbar}
-        {this.loading ? (
-          <div class={bem('loading')}>
-            <Loading />
-          </div>
-        ) : (
-          h()
-        )}
+        {this.loading ? <Loading class={bem('loading')} color={BLUE} /> : h()}
         <div class={bem('columns')} style={columnsStyle} onTouchmove={prevent}>
           {columns.map((item, index) => (
             <PickerColumn

packages/picker/index.less

@@ -43,12 +43,11 @@
     right: 0;
     bottom: 0;
     z-index: 2;
+    display: flex;
     position: absolute;
+    align-items: center;
+    justify-content: center;
     background-color: rgba(255, 255, 255, .9);
-
-    circle {
-      stroke: @blue;
-    }
   }
 
   &__loading .van-loading,

packages/picker/test/__snapshots__/demo.spec.js.snap

@@ -98,9 +98,7 @@ exports[`renders demo correctly 1`] = `
   </div>
   <div>
     <div class="van-picker">
-      <div class="van-picker__loading">
-        <div class="van-loading van-loading--circular" style="color:#c9c9c9;"><span class="van-loading__spinner van-loading__spinner--circular"><svg viewBox="25 25 50 50" class="van-loading__circular"><circle cx="50" cy="50" r="20" fill="none"></circle></svg></span></div>
-      </div>
+      <div class="van-loading van-loading--circular van-picker__loading"><span class="van-loading__spinner van-loading__spinner--circular" style="color:#1989fa;"><svg viewBox="25 25 50 50" class="van-loading__circular"><circle cx="50" cy="50" r="20" fill="none"></circle></svg></span></div>
       <div class="van-picker__columns" style="height:220px;">
         <div class="van-picker-column column1" style="height:220px;">
           <ul style="transition:0ms;transform:translate3d(0, 88px, 0);line-height:44px;">