为所有的输出资源添加版本号，以解决缓存问题。
class AssetVersioningPlugin {
apply(compiler) {
compiler.hooks.emit.tap('AssetVersioningPlugin', (compilation) => {
const version = new Date().getTime();
for (let filename in compilation.assets) {
compilation.assets[`${filename}?v=${version}`] = compilation.assets[filename];
delete compilation.assets[filename];
}
});
}
}