Async load webpack config

[kontrollanten]

I'm submitting a feature request

Current behavior:
karma-webpack will fail to parse the webpack property in the configuration when a the webpack config is is async loaded.

const webpackConfig = {
  entry: ...
}

module.exports = config => {
  config.set({
    webpack: new Promise(resolve => resolve(webpackConfig)),
  });
}

Expected/desired behavior:
Accept async configurations.

• What is the motivation / use case for changing the behavior?
  As Webpack 2+ has implemented this behavior Feature request: Async configuration via webpack.config.js webpack/webpack#2697, there's some modules that depends on it. I'm using electron-webpack, which only works with async configurations, and would like to use karma-webpack for my tests.

[langri-sha]

Using deasync in a pinch helps.