在webpack中使UglifyPlugin支持IE8

Uglify默认把screw_ie8默认为true，将screw_ie8配置为false:

new webpack.optimize.UglifyJsPlugin({
compress: {
        warnings: false,
        screw_ie8 : false
    },
    mangle: {
        screw_ie8: false
    },
    output: { screw_ie8: false }
})