Handle returned Promises

Webpack 2 supports returning a Promise from the config function.

https://webpack.js.org/configuration/configuration-types/#exporting-a-promise

This currently isn’t supported by webpack-merge.

[butterybread]

I have found a solution that does not require webpack-merge to support promises:

This does not work:

module.exports = merge(someConfig, async () => ({
  prop: await someFunc()
}));

This does work:

module.exports = async () => {
  const value = await someFunc();
  return merge(someConfig, { prop: value });
};

I am not sure if this solves every problem out there. But it solved mine.

[bebraw]

I will throw an error in this case at v5 (#135). It’s likely best to wrap merge within a Promise at the user space as that avoids complexity though I can still accept a PR at a later time.