message: [Improvement] Collapse: lazy render
diff: <cell :class="b('title')" is-link @click="onClick">
  <slot name="title">{{ title }}</slot>
  </cell>
  - <div v-show="show" ref="wrapper" :class="b('wrapper')" @transitionend="onTransitionEnd">
  + <div v-if="inited" v-show="show" ref="wrapper" :class="b('wrapper')" @transitionend="onTransitionEnd">
  <div ref="content" :class="b('content')">
  <slot />
  </div>
  @@ -33,7 +33,8 @@ export default create({
  data() {
  return {
  - show: null
  + show: null,
  + inited: null
  };
  },
  @@ -66,6 +67,7 @@ export default create({
  this.findParent('van-collapse');
  this.items.push(this);
  this.show = this.expanded;
  + this.inited = this.expanded;
  },
  destroyed() {
  @@ -80,6 +82,7 @@ export default create({
  if (expanded) {
  this.show = true;
  + this.inited = true;
  }
  this.$nextTick(() => {
  