add getProgress feature

[YanYuanFE]

_getProgress = (completedCount, total) => {
        const percent = (Math.round(completedCount/total * 10000)/100).toFixed(2) + '%'
        console.log(percent)
    }

<Preload

                images={images}
                onError={this._handleImageLoadError}
                onSuccess={this._handleImageLoadSuccess}
                getProgress={this._getProgress}
                mountChildren
                resolveOnError
            >

const propTypes = {
    // getProgress
    getProgress: PropTypes.func,
}

[diedu89]

@sambernard any news on this PR?

[YanYuanFE]

增加了加载图片获取进度的功能，修改了一些开发时的代码