message: [Improvement] Search: support native input attrs
diff: <input
  type="search"
  class="van-search__input"
  + v-bind="$attrs"
  v-refocus="focusStatus"
  :value="value"
  - :placeholder="placeholder"
  @input="onInput"
  @focus="onFocus"
  @keypress.enter.prevent="onSearch"
  >
  - <icon name="clear" @click="onClean" v-show="isFocus" />
  + <icon name="clear" v-show="isFocus && value" @click="onClean" />
  </div>
  <div class="van-search__action" v-if="showAction">
  <slot name="action">
  @@ -35,7 +35,6 @@ export default create({
  props: {
  value: String,
  showAction: Boolean,
  - placeholder: String,
  background: {
  type: String,
  default: '#f2f2f2'
  