[FEATURE] Searchbar input 中添加 placeholder-class，支持自定义 placeholder 的样式

[cloudman6]

https://github.com/wechat-miniprogram/weui-miniprogram/blob/master/src/components/searchbar/searchbar.wxml

希望 input 中增加一个 placeholder-class

        <input
            aria-controls="searchResult"
            type="text"
            class="weui-search-bar__input"
            placeholder-class="xxx"        <<===   支持自定义style
            placeholder="{{placeholder}}"
            value="{{value}}"
            focus="{{focus}}"
            bindblur="inputBlur"
            bindfocus="inputFocus"
            bindinput="inputChange"
        />